build-clan: fix bug where vars generator scripts are built for target system instead of local system

lib/build-clan/interface.nix

@@ -1,4 +1,8 @@
-{ lib, self, ... }:
+{
+  lib,
+  self,
+  ...
+}:
 let
   types = lib.types;
 in
@@ -71,13 +75,15 @@ in
 
     pkgsForSystem = lib.mkOption {
       type = types.functionTo (types.nullOr types.attrs);
-      default = _: null;
-      defaultText = "Lambda :: String -> { ... } | null";
+      default = _system: null;
+      defaultText = "system: null";
       description = ''
         A function that maps from architecture to pkg. `( string -> pkgs )`
 
         If specified this nixpkgs will be only imported once for each system.
-        This improves performance, but all nipxkgs.* options will be ignored.
+        This improves performance, but all `nipxkgs.*` options will be ignored.
+
+        Returning `null` for a system will fallback to the default behavior of respecting the `nixpkgs.*` options.
       '';
     };