nix: Build flakes as part of CI

.github/workflows/code.yaml

@@ -0,0 +1,21 @@
+name: "Code"
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+jobs:
+  code:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: DeterminateSystems/nix-installer-action@main
+        with:
+          extra-conf: |
+            trusted-public-keys = cache.garnix.io:CTFPyKSLcx5RMJKfLo5EEPUObbA78b0YQ2DTCJXqr9g= cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=
+            substituters = https://cache.garnix.io?priority=41 https://cache.nixos.org/
+      - uses: DeterminateSystems/magic-nix-cache-action@main
+      - uses: yaxitech/nix-install-pkgs-action@v3
+        with:
+          packages: "github:srid/nixci"
+      - run: nixci

flake.nix

@@ -76,5 +76,12 @@
           apps.default.program = self'.apps.en.program; # Alias to English site
           formatter = pkgs.nixpkgs-fmt;
         };
+      flake.nixci.default = {
+        nix-modules-1.dir = ./global/nix-modules/1;
+        nix-modules-2.dir = ./global/nix-modules/2;
+        nix-modules-3.dir = ./global/nix-modules/3;
+        nix-modules-4.dir = ./global/nix-modules/4;
+        nix-modules-5.dir = ./global/nix-modules/5;
+      };
     };
 }