nixos/default.nix: Allow specialArgs to be passed (#442886)

nixos/default.nix

@@ -1,12 +1,15 @@
 {
   configuration ? import ./lib/from-env.nix "NIXOS_CONFIG" <nixos-config>,
   system ? builtins.currentSystem,
+  # This should only be used for special arguments that need to be evaluated when resolving module structure (like in imports).
+  # For everything else, there's _module.args.
+  specialArgs ? { },
 }:
 
 let
 
   eval = import ./lib/eval-config.nix {
-    inherit system;
+    inherit system specialArgs;
     modules = [ configuration ];
   };