🔧 cancel previous CI runs if a new push is made (#1251)

Cancel previous CI runs if a new push is made. Thus saving a bit of CI
resources.

.github/workflows/build.yml

@@ -13,6 +13,10 @@ on:
       - master
   pull_request:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 jobs:
   api:
     runs-on: ubuntu-latest