chore: ci: add workflow for checking nix flake

Signed-off-by: Christoph Heiss <christoph@c8h4.io>

.forgejo/workflows/nix-flake.yaml

@@ -0,0 +1,26 @@
+---
+name: nix-flake
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    paths:
+      - '**/*.nix'
+
+env:
+  NIX_CONFIG: 'experimental-features = nix-command flakes'
+
+jobs:
+  test:
+    runs-on: nixos-unstable
+    steps:
+      - name: prepare nix container
+        run: nix-env -iA nixpkgs.nodejs
+
+      - name: checkout repository
+        uses: actions/checkout@v4
+
+      - name: flake check
+        run: nix flake check -L