Link to fixed tag for source links

[mkniewallner]

Closes #16882.

Make the "View Source" links point to a fixed tag version when TAG_NAME env var is set (which is the case in the release workflow) instead of always linking to master.

Test plan

Tested locally with both:

TAG_NAME=rust-1.95.0 cargo collect-metadata
cargo collect-metadata

Links point to https://github.com/rust-lang/rust-clippy/blob/rust-1.95.0/... in the first case, and https://github.com/rust-lang/rust-clippy/blob/master/... in the second one.

changelog: none

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