Use uv publish for 'publish' CI workflow

[khaeru]

Same as iiasa/ixmp#614.

How to review

Confirm the alpha release tagged on this branch is published to test.pypi.org.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation. NA, CI only
• Update release notes. ditto
• (after approval) Drop the TEMPORARY commit.
• (after merge) Remove the GitHub Actions secrets for PyPI and TestPyPI; delete the tokens on (Test)PyPI proper.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.2%. Comparing base (08cb547) to head (4b8dd46).
⚠️ Report is 3 commits behind head on main.

⚠️ Current head 4b8dd46 differs from pull request most recent head e6857d4

Please upload reports for the commit e6857d4 to get more accurate results.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #990   +/-   ##
=====================================
  Coverage   92.2%   92.2%           
=====================================
  Files         60      60           
  Lines       5155    5155           
=====================================
  Hits        4756    4756           
  Misses       399     399           

Files with missing lines	Coverage Δ
message_ix/cli.py	96.9% <ø> (ø)

[glatterf42]

LGTM :)