eval-config: set class

See [the Nixpkgs manual]. This allows modules to declare themselves
as being only for NixOS or nix-darwin, reducing compatibility
risks. Unmarked modules will continue to behave as before. Technically
a breaking change, but any configuration importing a module explicitly
marked as NixOS-specific was probably broken already.

[the Nixpkgs manual]: https://nixos.org/manual/nixpkgs/unstable/#module-system-lib-evalModules-param-class

eval-config.nix

@@ -16,6 +16,7 @@ let
   };
 
   eval = lib.evalModules (builtins.removeAttrs args [ "lib" ] // {
+    class = "darwin";
     modules = modules ++ [ argsModule ] ++ baseModules;
     specialArgs = { modulesPath = builtins.toString ./modules; } // specialArgs;
   });