modules: init nh for all hosts

modules/default.nix

@@ -1,5 +1,6 @@
-{...}: {
+{inputs, ...}: {
   imports = [
+    inputs.nh.nixosModules.default
     ./security.nix
     ./networking.nix
     ./users.nix

modules/nix.nix

@@ -1,10 +1,15 @@
-{pkgs, ...}: {
+_: {
   nixpkgs = {
     config = {
       allowUnfree = true;
     };
   };
 
+  nh = {
+    enable = true;
+    clean.enable = true;
+  };
+
   nix = {
     gc = {
       automatic = true;