Add flake check workflow

.github/workflows/check-flake.yml

@@ -0,0 +1,19 @@
+name: Flake Check
+
+on:
+  push:
+
+jobs:
+  flake-checks:
+    name: Check nix flake
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: cachix/install-nix-action@v27
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+
+      - name: Check flake
+        run: nix flake check