Only run PHPStan (on all PHP files) in pre-commit hook if there is a modified PHP file

[westonruter]

Summary

I found that our pre-commit hook was eagerly running PHPStan even when no modified PHP files were staged. We originally were going to include PHPStan in lint-staged via #5424 but reverted in #5438 since it's important for all PHP files to be included in the analysis. This PR conditionally runs PHPStan on all PHP files if any PHP file is modified.

Logic inspired by example in lint-staged docs:

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[github-actions]

Plugin builds for 225d5c0 are ready 🛎️!

• Download development build
• Download production build

[pierlon]

For some reason the "ESLint Report Analysis" check is not resolving albeit the "Annotate code linting results" GHA step runs without any errors.

I'm going to push a commit that resolves the warning being emitted from the GoogleCloudPlatform/github-actions/setup-gcloud@master action (see the "Annotations" section in the Actions summary) and see if the "ESLint Report Analysis" check manages to fix itself.

Actions Summary section (as of ad9fe69):

[pierlon]

Moving to the "QA Passed" column since there's nothing here to QA.