fix evaluation of darwin.pkgs for recent nixpkgs

The _module attribute was renamed in https://github.com/NixOS/nixpkgs/commits/dcdd232939232d04c1132b4cc242dd3dac44be8c.

default.nix

@@ -20,10 +20,14 @@ let
     specialArgs = { modulesPath = ./modules; };
     check = true;
   };
+
+  # Was moved in nixpkgs #82751, so both need to be handled here until 20.03 is deprecated.
+  # https://github.com/NixOS/nixpkgs/commits/dcdd232939232d04c1132b4cc242dd3dac44be8c
+  _module = eval._module or eval.config._module;
 in
 
 {
-  inherit (eval.config._module.args) pkgs;
+  inherit (_module.args) pkgs;
   inherit (eval) options config;
 
   system = eval.config.system.build.toplevel;