build: cancel in-progress runs on new commits (#42193)

.github/workflows/macos-build.yml

@@ -1,8 +1,14 @@
 name: Build MacOS
 
+# TODO: change to 'pull_request' and 'push' 
+# when we are ready to enable this for PRs.
 on:
   workflow_dispatch:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 env:
   GCLIENT_EXTRA_ARGS: '--custom-var=checkout_mac=True --custom-var=host_os=mac --custom-var=host_cpu=arm64'
   IS_RELEASE: false