Cancel in-progress builds upon EG. repeat push

Just being nice with Github by not needlessly taking up resources
running long jobs.

Author: badeball

.github/workflows/build.yml

@@ -2,6 +2,10 @@ name: Build
 
 on: [push, pull_request]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   prepare-versions:
     runs-on: ubuntu-latest