Update validation.yml

.github/workflows/validation.yml

@@ -38,8 +38,14 @@ jobs:
 
             - run: npm install
 
+            - name: Get modified files
+              if: github.event_name == 'pull_request'
+              id: files
+              run: |
+                    echo "::set-output name=files::$(git diff --name-only ${{ github.event.before }} ${{ github.sha }})"
+
             - run: npm test
               env:
                   EVENT: ${{ github.event_name }}
                   PR_AUTHOR: ${{ github.event.pull_request.user.login }}
-                  MODIFIED_FILES: ${{ env.MODIFIED_FILES }}
+                  MODIFIED_FILES: ${{ steps.files.outputs.files || '' }}