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);

nix/modules/default.nix

@@ -9,6 +9,15 @@
   ];
 
   options = {
+
+    nixpkgs = {
+      # TODO: switch to lib.systems.parsedPlatform
+      hostPlatform = lib.mkOption {
+        type = lib.types.str;
+        example = "x86_64-linux";
+      };
+    };
+
     assertions = lib.mkOption {
       type = lib.types.listOf lib.types.unspecified;
       internal = true;