Do not attempt to comment on PRs when on forks (#36316)

Summary:
I'm gating this workflow to run only upstream as I'm receiving notifications that it fails on my fork. We don't want to run this workflow on forks at all hence we can add a `if:` there.

## Changelog

[INTERNAL] - Do not attempt to comment on PRs when on forks

Pull Request resolved: https://github.com/facebook/react-native/pull/36316

Test Plan: If Github Actions UI is green, we're good to go

Reviewed By: cipolleschi

Differential Revision: D43657315

Pulled By: cortinico

fbshipit-source-id: 8fb260d142a43375bd9a3b10eac235550a6aecb0