fix: run nightly CI only on the main repo

[okwasniewski]

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

Test Plan:

CI Green

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	20,248,891	+0
android	hermes	armeabi-v7a	n/a	--
android	hermes	x86	n/a	--
android	hermes	x86_64	n/a	--
android	jsc	arm64-v8a	23,454,666	+2
android	jsc	armeabi-v7a	n/a	--
android	jsc	x86	n/a	--
android	jsc	x86_64	n/a	--

Base commit: 2483c63
Branch: main

[facebook-github-bot]

@cortinico has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@cortinico merged this pull request in 38bae4b.

[github-actions]

This pull request was successfully merged by @okwasniewski in 38bae4b.

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