CI: Cancel builds upon a new push

Signed-off-by: Stephen Finucane <[email protected]>

Author: stephenfin

.github/workflows/ci.yaml

@@ -3,6 +3,9 @@ name: CI
 on:
   - push
   - pull_request
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
 jobs:
   lint:
     name: Run linters