fix(workflows/update-mdn-urls): pass head_ref via env variable (#25054)

.github/workflows/update-mdn-urls.yml

@@ -33,11 +33,13 @@ jobs:
         run: npm ci
 
       - name: Update
+        env:
+          REF: ${{ github.head_ref }}
         run: |
           npm run lint:fix -- --only=mdn_urls
           if git diff --exit-code; then
             echo "No changes."
           else
             git commit -m 'chore: fix mdn urls' .
-            git push origin ${{ github.head_ref }}
+            git push origin "$REF"
           fi