cancel past CI runs on push

Author: Krastanov

.github/workflows/ci.yml

@@ -1,6 +1,12 @@
 name: CI
 env:
   JULIA_NUM_THREADS: 2
+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.ref != 'refs/heads/main') || github.run_number }}
+  # Cancel intermediate builds, but only if it is a pull request build.
+  cancel-in-progress: ${{ startsWith(github.ref, 'refs/pull/') }}
 on:
   pull_request:
     branches: