disable on PRs (#44912)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/44912

While moving from CircleCI → GHA, we're removing this blocking folks landing PRs and just running on main.  We will re-enable once GHA is stable.

Changelog: [General][Changed] Disable GHA on PRs until it's stable

Reviewed By: NickGerleman

Differential Revision: D58478000

fbshipit-source-id: 053ee53455956bf19b6f9113cb796346359ad4ef