Build/Test Tools: Allow the PHPUnit runner to be set by a repository variable.

[lancewillett]

Allows the PHPUnit runner to be directed by a repository variable, defaulting to the current runner.

reusable-phpunit-tests-v3.yml changes its runs-on to ${{ vars.PHPUNIT_RUNNER || inputs.os }}. With the PHPUNIT_RUNNER variable unset (the default), jobs run on inputs.os (ubuntu-24.04) exactly as before. When a maintainer sets PHPUNIT_RUNNER to a runner label, the PHPUnit matrices run on that runner instead—useful for directing them to a dedicated runner during high-load release windows, without editing the workflow.

• No behavior change by default; the variable is unset.
• Backward-compatible for every branch that calls this workflow @trunk; no input is added or made required.
• vars is available in runs-on, so the override is valid.

Trac: https://core.trac.wordpress.org/ticket/65749

[github-actions]

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[Copilot]

Pull request overview

This PR adds an optional GitHub Actions repository-variable override so maintainers can redirect the reusable PHPUnit workflow’s job runner without editing workflow files, while preserving the existing default behavior.

Changes:

• Updates the reusable PHPUnit workflow job to prefer vars.PHPUNIT_RUNNER (if set) over the existing inputs.os runner selection.
• Keeps backward compatibility by falling back to inputs.os when the variable is unset/empty.

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[lancewillett]

For this to work, @bazza or another GitHub admin will need to flip the PHPUNIT_RUNNER variable before / after release using GitHub.com settings for WordPress.org.

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How to route the PHPUnit matrices to a specific runner

This change is a no-op until a PHPUNIT_RUNNER repository variable exists. To direct the PHPUnit jobs to a runner (for example, a larger release-runner during a high-load release window):

⚠️ Note to admin: you'd need to do these steps separately per repo, wordpress-develop, wordpress-develop-security, and/or gutenberg (etc.)

Turn it on

1. Repo → Settings → Secrets and variables → Actions → Variables.
2. New repository variable → Name: PHPUNIT_RUNNER, Value: the runner label (e.g. release-runner).
3. New PHPUnit runs pick it up immediately. In-flight runs are unaffected; re-run if you need them moved.

Turn it off

1. Same page → delete the PHPUNIT_RUNNER variable.
2. PHPUnit jobs fall back to the default runner (ubuntu-24.04).

Notes

• Scoped per repository—set it only on the exact repo you want to redirect; other repos are unaffected.
• Larger hosted runners are billed per-minute, so set it only for the window and remove it after.
• Changing the variable needs repo admin access.

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 62891
GitHub commit: c5690fd

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.