nixos integration: add option 'useGlobalPkgs'

PR #1059

nixos/default.nix

@@ -15,6 +15,7 @@ let
         imports = import ../modules/modules.nix {
           inherit pkgs;
           lib = extendedLib;
+          useNixpkgsModule = !cfg.useGlobalPkgs;
         };
 
         config = {
@@ -45,6 +46,12 @@ in {
         <option>users.users.‹name?›.packages</option> option.
       '';
 
+      useGlobalPkgs = mkEnableOption ''
+        using the system configuration's <literal>pkgs</literal>
+        argument in Home Manager. This disables the Home Manager
+        options <option>nixpkgs.*</option>
+      '';
+
       backupFileExtension = mkOption {
         type = types.nullOr types.str;
         default = null;