refactor(nixpkgs): move nix settings into module

hosts/deertopia/configuration.nix

@@ -116,21 +116,6 @@
     };
   };
 
-  nix = {
-    settings = {
-      trusted-users = [
-        "@wheel"
-      ];
-      substituters = [
-        "https://nix-community.cachix.org"
-        "https://cache.nixos.org"
-      ];
-      trusted-public-keys = [
-        "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
-      ];
-    };
-  };
-
   # This option defines the first version of NixOS you have installed on this
   # particular machine, and is used to maintain compatibility with application
   # data (e.g. databases) created on older NixOS versions.