Skip concurrent CI builds (#7)

Committed via https://github.com/asottile/all-repos

Author: tobiasraabe

{{cookiecutter.project_slug}}/.github/workflows/main.yml

@@ -1,5 +1,11 @@
 name: Continuous Integration Workflow
 
+# Automatically cancel a previous run.
+concurrency:
+  group: ${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+
 on:
   push:
     branches: