GitHub Actions: Use uv cache, not pip cache

[cclauss]

Fix the failing GitHub Actions tests by using uv cache, instead of pip cache.

• https://docs.astral.sh/uv/guides/integration/github/#caching

A subset of:

• Upgrade to uv>=0.6.0 and enable-cache #2364 which also upgrades to uv v0.6.0 which contains breaking changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.16%. Comparing base (a255775) to head (fdf8f11).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2363   +/-   ##
=======================================
  Coverage   99.16%   99.16%           
=======================================
  Files          40       40           
  Lines        3098     3098           
  Branches      680      680           
=======================================
  Hits         3072     3072           
  Misses         15       15           
  Partials       11       11           

[DanielNoord]

Let's just merge #2364 instead :)

Thanks for giving us an option here!