fix github actions for forked PRs (#26562)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26562
I was trying to be too clever with GITHUB_HEAD_REF...
Test Plan: Imported from OSS
Reviewed By: driazati
Differential Revision: D17538517
Pulled By: suo
fbshipit-source-id: 82c71ee3c6edb299ac8eb73675d96967e00a29f1
