fix: run nightly CI only on the main repo (#44844)

Summary:
I've noticed that nightly CI build was also running on my fork. I don't think this is necessary for every React Native fork (there are 24k of forks). This can save lots of unnecessary CI time.

## Changelog:

[INTERNAL] [FIXED] - Enable nightly run only on the main repo

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Pull Request resolved: #44844

Test Plan: CI Green

Reviewed By: cipolleschi

Differential Revision: D58356192

Pulled By: cortinico

fbshipit-source-id: 1384d06708220d297e67d31433fcf3ac1d58bbbc

Author: okwasniewski

.github/workflows/nightly.yml

@@ -10,6 +10,7 @@ on:
 jobs:
   set_release_type:
     runs-on: ubuntu-latest
+    if: github.repository == 'facebook/react-native'
     outputs:
       RELEASE_TYPE: ${{ steps.set_release_type.outputs.RELEASE_TYPE }}
     env: