Add checkout step to check nightly (#47448)

Summary:
In order to use a reusable workflow in GHA, we need first to checkout the repository so the action has access to the other workflows.

## Changelog:
[Internal] - Add checkout step to Check Nightlies

Pull Request resolved: https://github.com/facebook/react-native/pull/47448

Test Plan: GHA - https://github.com/facebook/react-native/actions/runs/11702038686?pr=47448

Reviewed By: cortinico

Differential Revision: D65532274

Pulled By: cipolleschi

fbshipit-source-id: 58117ac81973e3c07829d73d5bde4e6fd4d212f9