CI: limit concurrency for PRs

Author: fingolfin

.github/workflows/CI.yml

@@ -5,6 +5,14 @@ on:
     branches:
       - release-0.18
     tags: '*'
+
+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 default repository branch we don't
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ github.ref_name != github.event.repository.default_branch || 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:
     name: Julia ${{ matrix.version }} - ${{ matrix.os }} - ${{ matrix.arch }} - ${{ github.event_name }}