modules: system: nix: preprend to 'NIX_PATH'

Instead of appending to the default value.

This makes overriding some values that are defined as the default value easier.

modules/system/nix/default.nix

@@ -35,11 +35,11 @@ in
     })
 
     (lib.mkIf cfg.addToNixPath {
-      nix.nixPath = options.nix.nixPath.default ++ [
+      nix.nixPath = [
         "self=${inputs.self}"
         "pkgs=${inputs.nixpkgs}"
         "nur=${inputs.nur}"
-      ];
+      ++ options.nix.nixPath.default;
     })
   ]);
 }