ci: add CI

Just runs `nix flake check`.

.forgejo/workflows/ci.yaml

@@ -0,0 +1,15 @@
+on: [push, pull_request]
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: https://github.com/cachix/install-nix-action@v26
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+          github_access_token: '${{ secrets.GH_TOKEN }}'
+      - uses: https://github.com/cachix/cachix-action@v14
+        with:
+          name: kitchen
+          authToken: '${{ secrets.CACHIX_TOKEN }}'
+      - run: nix flake check