CI: limit concurrent job on PRs

Author: wilfwilson

.github/workflows/CI.yml

@@ -8,9 +8,13 @@ on:
     # Every weekday at 01:30 AM UTC
     - cron: '30 1 * * 1-5'
 
-#concurrency:
-#  group: ${{ github.workflow }}-${{ github.ref }}
-#  cancel-in-progress: ${{ startsWith(github.ref, 'refs/pull/') }}
+# the `concurrency` settings ensure that not too many CI jobs run in parallel
+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: