Move mkHost helper into a separate module

lib/default.nix

@@ -0,0 +1,18 @@
+{ inputs }: {
+  mkHost = { hostName, system, timeZone, stateVersion }: inputs.nixpkgs.lib.nixosSystem {
+    inherit system;
+    modules = [
+      {
+        networking.hostName = hostName;
+        nix.settings.experimental-features = [ "nix-command" "flakes" ];
+        system.stateVersion = stateVersion;
+        time.timeZone = timeZone;
+      }
+      ../modules/nixos
+      ../hosts/${hostName}
+    ];
+    specialArgs = {
+      inherit inputs;
+    };
+  };
+}