Cancel all running build jobs on force-push of a PR branch

Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

.github/workflows/main.yml

@@ -6,6 +6,10 @@ on:
   pull_request:
     branches: [master]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 jobs:
   linux:
     runs-on: ubuntu-latest