fix(CI): run monitor-new-issues only in the main repo (#50951)

Summary:
This PR makes sure that monitor-new-issues runs only in the main repo. It was failing on my fork and spamming notifications.

## Changelog:

[INTERNAL] [FIXED] - run monitor-new-issues only in the main repo

Pull Request resolved: https://github.com/facebook/react-native/pull/50951

Test Plan: CI Green

Reviewed By: cipolleschi

Differential Revision: D73763006

Pulled By: cortinico

fbshipit-source-id: cbb22f8804448b41de970d3d11930110b353994f