Stop vale failing for forked contributions that don't touch prose. (#547)

Relates to 
- #549

(i.e. the prose lint failure for pull requests from forked
repositories).
Should improve the quality of life of external contributing devs.

This change _only_ annotates the diff introduced by a PR. So #549 will
still be a bug for external contributors who write a lot of prose. But
for technical contributions, at least, we avoid the 10 annotations
limit.

Author: samcunliffe

.github/workflows/linting.yml

@@ -71,5 +71,5 @@ jobs:
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         with:
-          filter_mode: nofilter
+          filter_mode: diff_context
           reporter: github-check