Allow the PHPUnit test runner to be set by a repository variable

[lancewillett]

The PHPUnit workflows run on a fixed runner (ubuntu-24.04), passed as the os input to reusable-phpunit-tests-v3.yml and consumed at runs-on: ${{ inputs.os }}.

This makes the runner overridable by a repository variable, so maintainers can direct the PHPUnit matrices to a specific runner, for example a dedicated runner during high-load release windows, without editing the workflow.

reusable-phpunit-tests-v3.yml changes its runs-on to:

runs-on: ${{ vars.PHPUNIT_RUNNER || inputs.os }}

With the PHPUNIT_RUNNER variable unset (the default), jobs run on inputs.os exactly as today, so there is no behavior change. It is backward-compatible for every branch that calls the workflow at @trunk; no input is added or made required.

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

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):

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.

[lancewillett]

In 62891:

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

Change runs-on in the reusable PHPUnit workflow to ${{ vars.PHPUNIT_RUNNER || inputs.os }}. With the variable unset (the default), jobs run 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 input is added or made required, so every branch that calls this workflow at @trunk stays compatible.

Developed in: ​https://github.com/WordPress/wordpress-develop/pull/12745

Props barry.
Fixes #65749.