Move most extra args out of eval-config.nix

nixos/modules/misc/extra-arguments.nix

@@ -0,0 +1,14 @@
+{ lib, pkgs, config, ... }:
+
+{
+  __internal.args = {
+    modulesPath = ../.;
+
+    pkgs_i686 = import ../../lib/nixpkgs.nix {
+      system = "i686-linux";
+      config.allowUnfree = true;
+    };
+
+    utils = import ../../lib/utils.nix pkgs;
+  };
+}

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;
+    };
+  };
 }