build: cancel previous github action runs (#11940)

.github/workflows/superset-python-mysql.yml

@@ -28,6 +28,10 @@ jobs:
         ports:
           - 16379:6379
     steps:
+      - name: Cancel Previous Runs
+        uses: styfle/cancel-workflow-action@0.6.0
+        with:
+          access_token: ${{ github.token }}
       - uses: actions/checkout@v2
       - name: Setup Python
         uses: actions/setup-python@v2