ci: bump actions/checkout from v5 to v6

Every workflow currently pins `actions/checkout` to v5, which was
introduced primarily to move to the Node.js 24 runtime. v6 is the
next release and worth picking up so we stay on a maintained version
of the action.

The one behaviorally interesting change in v6:

  `persist-credentials` now stores the helper credentials under
  `$RUNNER_TEMP` instead of writing them directly into the local
  `.git/config`. Two implications follow:

  1. In the normal case this is an unambiguous improvement -- the
     token no longer lands in `.git/config`, reducing the risk of
     inadvertently leaking it through workspace archiving
     (`upload-artifact` snapshots, cache entries, core dumps, ...).

  2. Docker container actions require an Actions Runner of at least
     v2.329.0 to find the credentials in their new location. The
     github.com-hosted runners our CI uses are already past that
     version, so this does not affect us. Downstream users running
     self-hosted runners may need to update them before adopting
     this version of the action.

Risk analysis: our checkout steps either check out the default
repository (no special credential requirements) or, in the `vs-build`
job, explicitly set `repository: microsoft/vcpkg` and
`path: compat/vcbuild/vcpkg`. Neither case relies on the precise
location of the persisted credentials -- subsequent steps interact
with the API via the runner-provided `GITHUB_TOKEN` directly -- so
the v6 credential-storage change is transparent to our workflows.
The diff is purely the `@vN` identifier; there are no input or
output changes.

See also:

- Release notes: https://github.com/actions/checkout/releases
- Changelog: https://github.com/actions/checkout/blob/main/CHANGELOG.md
- Compare: actions/checkout@v5...v6

Originally-authored-by: dependabot[bot] <support@github.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

.github/workflows/check-style.yml

@@ -20,7 +20,7 @@ jobs:
       jobname: ClangFormat
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         fetch-depth: 0
 

.github/workflows/check-whitespace.yml

@@ -19,7 +19,7 @@ jobs:
   check-whitespace:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         fetch-depth: 0
 

.github/workflows/coverity.yml

@@ -38,7 +38,7 @@ jobs:
       COVERITY_LANGUAGE: cxx
       COVERITY_PLATFORM: overridden-below
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: install minimal Git for Windows SDK
         if: contains(matrix.os, 'windows')
         uses: git-for-windows/setup-git-for-windows-sdk@v1

.github/workflows/main.yml

@@ -112,7 +112,7 @@ jobs:
       group: windows-build-${{ github.ref }}
       cancel-in-progress: ${{ needs.ci-config.outputs.skip_concurrent == 'yes' }}
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: git-for-windows/setup-git-for-windows-sdk@v1
     - name: build
       shell: bash
@@ -173,10 +173,10 @@ jobs:
       group: vs-build-${{ github.ref }}
       cancel-in-progress: ${{ needs.ci-config.outputs.skip_concurrent == 'yes' }}
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: git-for-windows/setup-git-for-windows-sdk@v1
     - name: initialize vcpkg
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
       with:
         repository: 'microsoft/vcpkg'
         path: 'compat/vcbuild/vcpkg'
@@ -258,7 +258,7 @@ jobs:
       group: windows-meson-build-${{ github.ref }}
       cancel-in-progress: ${{ needs.ci-config.outputs.skip_concurrent == 'yes' }}
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: actions/setup-python@v6
     - name: Set up dependencies
       shell: pwsh
@@ -286,7 +286,7 @@ jobs:
       group: windows-meson-test-${{ matrix.nr }}-${{ github.ref }}
       cancel-in-progress: ${{ needs.ci-config.outputs.skip_concurrent == 'yes' }}
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: actions/setup-python@v6
     - name: Set up dependencies
       shell: pwsh
@@ -341,7 +341,7 @@ jobs:
       TEST_OUTPUT_DIRECTORY: ${{github.workspace}}/t
     runs-on: ${{matrix.vector.pool}}
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - run: ci/install-dependencies.sh
     - run: ci/run-build-and-tests.sh
     - name: print test failures
@@ -362,7 +362,7 @@ jobs:
       CI_JOB_IMAGE: ubuntu-latest
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - run: ci/install-dependencies.sh
     - run: ci/run-build-and-minimal-fuzzers.sh
   dockerized:
@@ -439,7 +439,7 @@ jobs:
         else
           apt-get -q update && apt-get -q -y install git
         fi
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - run: ci/install-dependencies.sh
     - run: useradd builder --create-home
     - run: chown -R builder .
@@ -464,7 +464,7 @@ jobs:
       group: static-analysis-${{ github.ref }}
       cancel-in-progress: ${{ needs.ci-config.outputs.skip_concurrent == 'yes' }}
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - run: ci/install-dependencies.sh
     - run: ci/run-static-analysis.sh
     - run: ci/check-directional-formatting.bash
@@ -480,7 +480,7 @@ jobs:
       group: rust-analysis-${{ github.ref }}
       cancel-in-progress: ${{ needs.ci-config.outputs.skip_concurrent == 'yes' }}
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - run: ci/install-dependencies.sh
     - run: ci/run-rust-checks.sh
   sparse:
@@ -494,7 +494,7 @@ jobs:
       group: sparse-${{ github.ref }}
       cancel-in-progress: ${{ needs.ci-config.outputs.skip_concurrent == 'yes' }}
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Install other dependencies
       run: ci/install-dependencies.sh
     - run: make sparse
@@ -510,6 +510,6 @@ jobs:
       CI_JOB_IMAGE: ubuntu-latest
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - run: ci/install-dependencies.sh
     - run: ci/test-documentation.sh