ci: count an unwritable Homebrew prefix as a machine failure

[teamleaderleo]

Summary

A macOS runner whose /opt/homebrew is not owned by the runner user fails in the test step's Homebrew install, before any test starts: "The following directories are not writable by your user". Run 36356259693 hit it on one owned Mac. The code under test cannot cause this, so scripts/ci/machine_failure.py now lists it with the other runner-level signatures. The focused dispatcher then retries it automatically, and the regression bisect counts it as an error, not a failing commit.

The Mac itself still needs its Homebrew prefix fixed (chown -R to the runner user). This change only stops the error from being read as a test result.

Testing

tests/test_ci_machine_failure.py adds a case for the Homebrew message. All cases pass locally.

Changelog

none

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Summary by cubic

Counts a macOS runner whose Homebrew prefix isn't writable by the runner user as a machine failure instead of a product failure, so the focused dispatcher retries it and the regression bisect no longer blames a commit. Adds a test case for the Homebrew error message.

^{Written for commit 04aad34. Summary will update on new commits.}

Summary by CodeRabbit

• Bug Fixes
  • Machine failures caused by an unwritable Homebrew prefix are now reported with a clear explanation.

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📥 Commits

Reviewing files that changed from the base of the PR and between 51aad0b and 04aad34.

📒 Files selected for processing (2)

• scripts/ci/machine_failure.py
• tests/test_ci_machine_failure.py

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[github-actions]

Merge receipt for 04aad3474d: every check was green at merge (13 verified; 18 skipped by policy). Full suite runs on main after merge.