tests.yml: on an Ubuntu 26.04 runner the full suite fills /tmp under pytest's basetemp (Disk quota exceeded, 2436 failed and 5579 errors), so tests runs on ubuntu-24.04 until the suite can run there #1096

Found by lane 3 on #1092's CI run. It follows #1094, whose PR #1095 now moves the tests job to ubuntu-24.04 until the suite can run on Ubuntu 26.04.

What happens. On an Ubuntu 26.04 runner, the full suite fills the space under pytest's basetemp in /tmp, and the run collapses.

• The run: shape-doctor.py: a leg's gitlink that no .gitmodules section registers, beside one whose sections register other paths or beside none, is named with its own way out, as #914 names one beside a .gitmodules with no section (#1080) #1092's tests job (job 119050702775, image ubuntu-26.04 version 20261012.163.1, Ubuntu 26.04.1 LTS) ended at 06:01:58Z with 2436 failed, 11382 passed, 380 skipped, 5579 errors in 3048.23s.
• The log has 28325 lines with [Errno 122] Disk quota exceeded, and later ones with No space left on device. All of them are under /tmp/pytest-of-runner/pytest-0.
• The failures are spread over unrelated files, for example test_dry_run_parity.py, test_family_holder_bootstrap_texts.py and test_validate_pins_without_git.py.
• The same suite passed on ubuntu-latest while it still gave out 24.04, for example main's push run at 01:27Z.
• The Disk quota exceeded points to a /tmp with a quota on the 26.04 image, unlike 24.04's /tmp on the runner's disk. That is an inference and has not been measured.

Why it matters. ubuntu-latest is moving to 26.04. While tests stays pinned to 24.04, the suite never runs on the image the release's install check uses for its 3.x rows on ubuntu-latest.

Ask.

• Point pytest's basetemp, and TMPDIR for everything the tests start, at the runner's disk: ${{ runner.temp }}, or the workspace.
• Run the whole suite once on ubuntu-26.04 to see what else differs. The 26.04 image's git is 2.55.0, and the doctor's docstrings were measured with git 2.43.
• Then move tests back to ubuntu-latest, and drop the pin tests.yml and release.yml: the full suite and the Python 3.9 jobs run on ubuntu-24.04, since the Ubuntu 26.04 that ubuntu-latest now gives out has no 3.9 for setup-python and runs out of /tmp under the suite (#1094) #1095 adds.

Lane: openreposhape-3 (openRepoShape-3)