Fix flaky run_actionlint! tests

[MikeMcQuaid]

• Seed $CHILD_STATUS with system("true") before stubbing so it is non-nil when the code calls $CHILD_STATUS.success? after the stubbed system

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[Copilot]

Pull request overview

Stabilizes the Homebrew::Style.run_actionlint! specs by ensuring $CHILD_STATUS is initialized before system is stubbed, preventing nil.success? failures.

Changes:

• Seed $CHILD_STATUS via a trivial successful system("true") call in the run_actionlint! spec setup.

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