fix: add Python 3.15 and update OS versions in tests.yml

[cclauss]

fix: add Python 3.15 and update OS versions in tests.yml

Failures on Python 3.15 release candidate 2. The production release is scheduled for Oct. 1st.

• https://peps.python.org/pep-0790

Related to the failures at:

• fix(ci): enable Python 3.15 release candidate on Windows gyp-next#357

Checklist

• npm install && npm run lint && npm test passes
• tests are included
• documentation is changed or added
• commit message follows commit guidelines

Description of change

[gengjiawen]

Opened #3370 to fix the LNK1104 failures in the Windows/Python 3.15 jobs here. The non-ASCII addon test currently uses --nodedir with a downloaded SDK cache, selecting Release/node.lib instead of <arch>/node.lib. Python 3.15's default UTF-8 mode exposes this existing mismatch by allowing the previously skipped Latīna test to run.

The fix links the cache root and uses --devdir, preserving the non-ASCII path. Targeted Windows validation of this path change on Node.js 24 passed all 3 addon tests without skips with both Python 3.14 (PYTHONUTF8=1) and Python 3.15 defaults; #3370 documents the tested revision.

After #3370 lands, update this branch to include the fix and run the full CI matrix again to confirm the Python 3.15 failures are resolved.