CI: Cancel outdated PR builds when new commits are pushed to a PR branch (#47)

Author: DilumAluthge

.github/workflows/CI.yml

@@ -5,6 +5,11 @@ on:
       - master
     tags: '*'
   pull_request:
+concurrency:
+  # Skip intermediate builds: all builds except for builds on the `master` branch
+  # Cancel intermediate builds: only pull request builds
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.ref != 'refs/heads/master' || github.run_number }}
+  cancel-in-progress: ${{ startsWith(github.ref, 'refs/pull/') }}
 jobs:
   test:
     runs-on: ${{ matrix.os }}