Move most extra args out of eval-config.nix

nixos/modules/misc/nixpkgs.nix

@@ -60,6 +60,7 @@ in
 
     nixpkgs.system = mkOption {
       type = types.str;
+      default = builtins.currentSystem;
       description = ''
         Specifies the Nix platform type for which NixOS should be built.
         If unset, it defaults to the platform type of your host system.
@@ -69,4 +70,12 @@ in
     };
 
   };
+
+  config = {
+    __internal.args.pkgs = import ../../lib/nixpkgs.nix {
+      system = config.nixpkgs.system;
+
+      inherit (config.nixpkgs) config;
+    };
+  };
 }