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: #50951

Test Plan: CI Green

Reviewed By: cipolleschi

Differential Revision: D73763006

Pulled By: cortinico

fbshipit-source-id: cbb22f8804448b41de970d3d11930110b353994f

Author: okwasniewski

.github/workflows/monitor-new-issues.yml

@@ -12,6 +12,7 @@ on:
 jobs:
   monitor-issues:
     runs-on: ubuntu-latest
+    if: github.repository == 'facebook/react-native'
     steps:
       - name: Checkout
         uses: actions/checkout@v4