Cancel queued/running builds on second push to PR (#9050)

This uses an action from the marketplace to cancel any running builds
for our main "CI" workflow (the only one we have at the moment)

.github/workflows/cancel.yml

@@ -0,0 +1,30 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+name: Cancel
+on: [push]
+jobs:
+  cancel:
+    name: 'Cancel Previous Runs'
+    runs-on: ubuntu-latest
+    timeout-minutes: 3
+    steps:
+      - uses: styfle/cancel-workflow-action@0.3.2
+        with:
+          # Fetch via https://api.github.com/repos/apache/airflow/actions/workflows/ci.yml
+          workflow_id: 1029499
+          access_token: ${{ github.token }}