github-ci: cancel previous nix workflow for branch

On push, cancel previous nix workflows for the same branch.

Author: jasonish

.github/workflows/nix.yml

@@ -2,6 +2,9 @@ name: "Nix Env Build"
 on:
   pull_request:
   push:
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
 jobs:
   tests:
     runs-on: ubuntu-latest