lib/builders.nix: add default nixpkgs hostplatform to mkSystem

parts/lib/builders.nix

@@ -1,6 +1,8 @@
 {inputs, ...}: let
   inherit (inputs) self nixpkgs;
   inherit (nixpkgs) lib;
+  inherit (lib) mkDefault nixosSystem recursiveUpdate singleton;
+  inherit (builtins) concatLists;
 in {
   mkSystem = {
     withSystem,
@@ -13,17 +15,27 @@ in {
         self',
         ...
       }:
-        lib.nixosSystem {
+        nixosSystem {
           inherit system;
           specialArgs =
-            lib.recursiveUpdate
+            recursiveUpdate
             {
               inherit lib;
               inherit inputs inputs';
               inherit self self';
             }
             (args.specialArgs or {});
-          inherit (args) modules;
+          modules = concatLists [
+            # This is used to pre-emptively set the hostPlatform for nixpkgs.
+            # We need a singleton here since we concatenate lists, and a singleton
+            # generates a list with a single element.
+            (singleton {
+              nixpkgs.hostPlatform = mkDefault args.system;
+            })
+
+            # We pass our modules from the arguments
+            (args.modules or [])
+          ];
         }
     );
 }

parts/lib/default.nix

@@ -44,6 +44,6 @@ in {
   # `flake.lib` is set.
   flake = {
     lib = extendedLib;
-    _module.args.lib = extendedLib;
+    # _module.args.lib = extendedLib;
   };
 }