CI: test-crc32c and test-zarr run on system Python — matrix.python-version is never defined #855

Minimal, reproducible code sample, a copy-pastable example if possible

Any green test-crc32c or test-zarr job log shows it — e.g. the latest main run of Tests (27815631459):

##[warning]The `python-version` input is not set.  The version of Python currently in `PATH` will be used.
...
platform linux -- Python 3.12.3, pytest-9.1.0, pluggy-1.6.0 -- /usr/bin/python

(Same on the newest executed run, 2026-08-03.)

Problem description

The test-crc32c and test-zarr jobs pass python-version: ${{ matrix.python-version }} to actions/setup-python (L79, L129), but neither job's matrix defines a python-version key — only the build job's matrix does. With an empty input, setup-python falls back to the runner's PATH Python, so all six variants of these two jobs — including the zarr @ main integration entry — run on the ubuntu-24.04 image's /usr/bin/python 3.12.3 rather than a version the workflow controls: they never see the 3.13/3.14 from the build matrix, and they'll change interpreter again whenever ubuntu-latest rolls forward.

This came in with the Conda removal in #839, which replaced the pinned python-version: "3.13" in both jobs with the matrix expression used by the build job.

Happy to send the two-line PR — an explicit pin restores the pre-#839 behavior, or python-version can be added to both matrices if version × variant coverage is wanted; whichever you prefer.

Found while auditing workflow runs with AI assistance; the yaml, the #839 diff, and the job logs quoted above were each re-checked by hand.

Version and installation information

N/A — CI configuration on main (82c3635).