Remove unstable uv lock from pip interface

[charliermarsh]

Summary

I think we can start using uv lock and uv sync to test this instead.

[codspeed-hq]

CodSpeed Performance Report

Merging #3970 will not alter performance

_{Comparing charlie/unstable (9aba38d) with main (fbf562d)}

Summary

✅ 13 untouched benchmarks

[BurntSushi]

The PR itself looks good. I think one remaining thing here (because I just tried it) is specifying the index URL. I can do that with uv pip compile, but I'm not sure how to do that with uv lock yet. We need to be able to specify an index URL for packse tests.

[BurntSushi]

I can confirm that I can run packse tests on the universal resolver without the use of --unstable-uv-lock-file, so this should be good to go. 👍 Thank you!