ci: waste less cpu cycles

When a pull request is force-pushed to so that a new job is created,
cancel the previous running job.

Author: NickeZ

.github/workflows/pr-ci.yml

@@ -3,6 +3,9 @@
 
 name: Pull request CI
 on: pull_request
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
 
 jobs:
   pr-head-ci: