GitHub Actions: Fix PHP file change detection logic

[t-hamano]

This issue was discovered by a bot comment below:

#59531 (comment)

What?

This PR fixes an issue where GitHub Actions that prompt PHP backports to core could unintentionally detect non-PHP files.

Why?

The lib and phpunit directories contain more than just php files.

Testing Instructions

This regular expression should be correct, but you can test it by modifying and pushing the files in the lib, phpunit directories in this PR.

[github-actions]

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Co-authored-by: t-hamano <[email protected]>
Co-authored-by: ramonjd <[email protected]>
Co-authored-by: desrosj <[email protected]>

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[ramonjd]

Catching all files was intentional just in case there were any .md files or other items that might need a Core patch, but it is a bit of an overkill.

This LGTM

🙇🏻

[desrosj]

Thanks @t-hamano!

[github-actions]

Warning: Type of PR label mismatch

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