CI: uutils timeout on Ubuntu 26.04 runners breaks --kill-after grace (main red) #2020

Describe the bug
On ubuntu-latest runners that now get Ubuntu 26.04, timeout is the uutils Rust rewrite (0.10.0) rather than GNU coreutils. Its --kill-after sends KILL right after the first signal, with no grace period, and sends it only to the direct child, not to the process group. As a result:

• main d419d8c39 is red on "Plain unittest ordering (project)", and ci: run the curated ALL tier on PyPy, split into separate pytest runs (#1958, #2005) #2006's Project tests fail the same 3 tests: test_resource_monitor_1740::test_a_timeout_exits_124, ::test_a_tests_step_killed_after_the_timeout_records_137_and_its_time, and test_graalpy_shards_1963::test_the_timeout_ends_the_run_in_progress_and_starts_no_other. They see 137 where 124 is expected.
• The real tests step (timeout --signal=INT --kill-after=3m 110m in experimental.yml) loses pytest's teardown grace on 26.04, and its xdist workers are left orphaned.

Reproduction

• Failing jobs ran on ubuntu-26.04 (image 20261018.168.1): 119423889806 on main and 119422726301 on ci: run the curated ALL tier on PyPy, split into separate pytest runs (#1958, #2005) #2006.
• Passing jobs ran on ubuntu-24.04, including job 119413449722 on the same sha d419d8c39.
• In docker run ubuntu:26.04, timeout --version reports uutils coreutils 0.10.0. Running timeout -v --signal=INT --kill-after=3s 1 sleep 30 gives exit 137 after about 1 s. gnutimeout, which is GNU 9.7 on the same image, gives 124.

Expected behavior
The timeout wrappers behave as they do with GNU timeout on either image.

System information

• OS Version: Ubuntu 26.04 (GitHub Actions ubuntu-latest)
• commit: d419d8c39

Additional context

• Same rollout as CI: tshark can't read captures on Ubuntu 26.04 runners (AppArmor profile) #2019, different mechanism.
• Proposed fix: the step scripts use gnutimeout when it exists and fall back to timeout otherwise. The tests replay those scripts, so they pick the fix up too. Pinning ubuntu-24.04 only buys time.
• Confirmed on the Docker image; not yet confirmed on the runner image itself.