Add pre-push hook check to block leftover .only()s

[charisk]

Checklist

N/A:

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[shati-patel]

Thanks! This works fine for me (using Git Bash on Windows), though I'll let someone else take a look too 🌷

[aeisenberg]

Thanks for fixing this.