Pin Percy to latest commit on the target branch

Author: querkmachine

.github/workflows/screenshots.yml

@@ -15,7 +15,8 @@ jobs:
 
     env:
       PERCY_PARALLEL_NONCE: ${{ github.run_id }}-${{ github.run_attempt }}
-      PERCY_TARGET_BRANCH: ${{ github.base_ref }}
+      PERCY_TARGET_BRANCH: ${{ github.event.pull_request.base.ref }}
+      PERCY_TARGET_COMMIT: ${{ github.event.pull_request.base.sha }}
       PERCY_TOKEN: ${{ secrets.PERCY_TOKEN }}
 
     # Skip when secrets are unavailable on forks