Add concurrency to workflow

Author: Alex Westergaard

.github/workflows/ci.yml

@@ -5,6 +5,10 @@ on:
     branches:
       - "master"
 
+concurrency:
+  group: ${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
 jobs:
   validate-mergable:
     runs-on: ubuntu-latest