Use concurrency (#3610)

Use concurrency in GitHub workflows to cancel in-progress or pending
runs for the same ref.

.github/workflows/macos.yml

@@ -9,6 +9,10 @@ on:
   pull_request:
   workflow_dispatch:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref || github.run_id }}
+  cancel-in-progress: true
+
 jobs:
   xcode_1:
     runs-on: macos-10.15