Cancel in progress CI jobs for PRs

.github/workflows/test.yml

@@ -8,6 +8,13 @@ on:
   pull_request:
   workflow_dispatch:
 
+concurrency:
+  # group by workflow and ref; the last slightly strange component ensures that for pull
+  # requests, we limit to 1 concurrent job, but for the master branch we don't
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.ref != 'refs/heads/master' || github.run_number }}
+  # Cancel intermediate builds, but only if it is a pull request build.
+  cancel-in-progress: ${{ startsWith(github.ref, 'refs/pull/') }}
+
 jobs:
   test:
     runs-on: ${{ matrix.os }}