Fix pull request target (#3676)

Co-authored-by: Saaketh Narayan <[email protected]>

.github/workflows/pr-gpu.yaml

@@ -1,6 +1,6 @@
 name: PR GPU tests
 on:
-  pull_request:
+  pull_request_target:
   workflow_dispatch:
 # Cancel old runs when a new commit is pushed to the same branch if not on main
 # or dev