Fix cache-prune-test to work outside the git repository (#4375)
## Summary
Make the git commit/date part of the version string matched in
cache_prune optional, so that the test also works correctly when uv is
built from an unpacked release tarball rather than a git repository.
Fixes #4374
## Test Plan
Ran `cargo test` inside the git repository and inside unpacked archive
for `0.2.12` release with the patch applied on top.
