feat(ci): add concurrency (#38)

This makes it so that inprogress builds are cancelled when you make a new push. Matches the rest of ublue.

.github/workflows/build.yml

@@ -18,6 +18,10 @@ env:
   MY_IMAGE_DESC: "My Customized Universal Blue Image"
   IMAGE_REGISTRY: "ghcr.io/${{ github.repository_owner }}"  # do not edit
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref || github.run_id }}-${{ inputs.brand_name}}-${{ inputs.stream_name }}
+  cancel-in-progress: true
+
 jobs:
   build_push:
     name: Build and push image