fix(ci): keep coverage profraw writable under umask 0777 tests

[l46983284-cpu]

Summary

Code Coverage can fail exact-stderr tests (for example mkfifo) when LLVM prints:

LLVM Profile Error: Failed to write file "...profraw": Permission denied

Root cause is not the utility under test. Some uutests set umask(0777) before spawning the binary. LLVM profile files created under that umask can be mode 000; later profile writes fail with EACCES. That message lands on the utility stderr and breaks stderr_is asserts. A shared %4m profile pool made this worse across processes.

This PR fixes the coverage harness:

1. prepare_profraw_dir recreates the trace directory with a default owner-write ACL via setfacl (hard-fail on CI if setfacl is missing or fails).
2. LLVM_PROFILE_FILE uses coverage-%p-%m.profraw instead of a shared %4m pool.
3. The Ubuntu Code Coverage job installs the acl package so setfacl is present.

This replaces the earlier stderr-filter approach on this same PR. A capture-side filter can still be useful later as defense-in-depth for other LLVM I/O noise, but is not this change.

Test plan

• Local mechanism canary: create under umask 0777 → mode 000, reopen fails; with default ACL → mode 660, reopen succeeds
• Script path only: util/build-run-test-coverage-linux.sh + coverage job apt install
• CI Code Coverage green on this branch
• No change to product utilities or uutests product umask behavior

Notes

• Does not weaken umask tests.
• Does not strip LLVM diagnostics from captured stderr (optional belt in fix(ci): keep coverage profraw writable under umask 0777 tests #13502).

[sylvestre]

This does not repair the underlying coverage profile-file permission failure;

did you try to fix the issue itself instead ?

[l46983284-cpu]

Yes, now — that was only a stderr filter. I replaced this PR with a coverage-script fix for the umask/profraw issue itself.

[codspeed-hq]

Merging this PR will not alter performance

✅ 339 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing l46983284-cpu:fix/uutests-llvm-profile-stderr (6c62e75) with main (decd874)}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/retry (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)

[sylvestre]

much better, thanks

[RenjiSann]

Great ! thanks :)