Add annotations to PRs from forks (#54779)
Summary:
We've been using [pytorch/add-annotations-github-action](https://github.com/pytorch/add-annotations-github-action) to add annotations to PRs when they fail Flake8 or clang-tidy. Up until now, though, that functionality has only worked on PRs in pytorch/pytorch itself, not on PRs from forks. This PR fixes that using a technique from [this GitHub blog post](https://securitylab.github.com/research/github-actions-preventing-pwn-requests/) (also linked in a comment in this diff).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54779
Test Plan: janeyx99 and I tested this in the same GitHub repo used to test https://github.com/pytorch/pytorch/issues/54685 and https://github.com/pytorch/pytorch/issues/54693, including with PRs from forks.
Reviewed By: walterddr
Differential Revision: D27364777
Pulled By: samestep
fbshipit-source-id: a830d372d7bb3b2529fc633b707b44f2b6cf9baa
