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: https://github.com/facebook/react-native/pull/44844

Test Plan: CI Green

Reviewed By: cipolleschi

Differential Revision: D58356192

Pulled By: cortinico

fbshipit-source-id: 1384d06708220d297e67d31433fcf3ac1d58bbbc