fix: use head_ref instead of head_sha

.github/workflows/update_regression_baseline.yml

@@ -25,7 +25,7 @@ jobs:
     - name: Checkout PR branch
       uses: actions/checkout@v3
       with:
-        ref: ${{ steps.comment-branch.outputs.head_sha }}
+        ref: ${{ steps.comment-branch.outputs.head_ref }} # using head_sha vs. head_ref makes this work for forks
         lfs: true
         fetch-depth: 0 # This ensures we can checkout main branch too
 
@@ -51,4 +51,4 @@ jobs:
       run: |
         git add tests/regression_test_baselines.json
         git commit -m "Update regression test baselines"
-        git push origin HEAD:${{ steps.comment-branch.outputs.head_sha }}
+        git push origin HEAD:${{ steps.comment-branch.outputs.head_ref }}