ci: run cache-repear only on main repo (#45940)

Summary:
This PR enables the `cache-repear.yml` only for the main repository. This is running constantly on forks creating lots of notifications and it's mostly needed only for the main repo.

![CleanShot 2024-08-08 at 10 06 38@2x](https://github.com/user-attachments/assets/51f7e208-05c9-4a02-982d-d746a717dc69)

## Changelog:

[INTERNAL] [CHANGED] - Run `cache-repear.yml` only on main repo

Pull Request resolved: https://github.com/facebook/react-native/pull/45940

Test Plan: CI GREEN

Reviewed By: blakef

Differential Revision: D60957003

Pulled By: cortinico

fbshipit-source-id: 2f250d734688739a278095af8d860a54426604bf