Set the nixpkgs platform as a module option.

nix/lib.nix

@@ -8,8 +8,7 @@ let
 in
 {
   makeSystemConfig =
-    { system
-    , modules
+    { modules
     , extraSpecialArgs ? { }
     ,
     }:
@@ -34,6 +33,8 @@ in
         ] ++ modules;
       }).config;
 
+      system = config.nixpkgs.hostPlatform;
+
       returnIfNoAssertions = drv:
         let
           failedAssertions = map (x: x.message) (lib.filter (x: !x.assertion) config.assertions);