Update format checker action (microsoft#6067)

Updates the format checker action yaml to conform to our policies

Author: Greg Roth

.github/workflows/clang-format-checker.yml

@@ -26,9 +26,10 @@ jobs:
           fetch_depth: 100 # Fetches only the last 10 commits
 
       - name: "Listed files"
+        env: LISTED_FILES: ${{ steps.changed-files.outputs.all_changed_files }}
         run: |
           echo "Formatting files:"
-          echo "${{ steps.changed-files.outputs.all_changed_files }}"
+          echo "$LISTED_FILES"
 
       - name: Install clang-format
         uses: aminya/setup-cpp@v1