Add nix flake check CI

Author: rhelmot

.github/workflows/test.yml

@@ -0,0 +1,13 @@
+name: "nix flake check"
+on:
+  pull_request:
+  push:
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: cachix/install-nix-action@v31
+      #with:
+      #  nix_path: nixpkgs=channel:nixos-unstable
+    - run: nix --extra-experimental-features 'nix-command flakes' flake check