Add "cancel-in-progress" option for Nix runner.

.github/workflows/nix.yml

@@ -2,6 +2,12 @@ name: "Nix CI"
 
 on: [pull_request, push]
 
+# https://stackoverflow.com/questions/66335225/how-to-cancel-previous-runs-in-the-pr-when-you-push-new-commitsupdate-the-curre#comment133398800_72408109
+# https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#concurrency
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref || github.run_id }}
+  cancel-in-progress: true
+
 jobs:
   tests:
     name: "Nix build on ${{ matrix.os }}"