add resulting Nix value to default.nix outputs

this is practical for debugging and demonstration purposes

default.nix

@@ -15,11 +15,8 @@ let
     new // { types = prev.recursiveUpdate prev.types new.types; };
   lib'' = lib.extend lib';
 in
-{
+rec {
   lib = import ./lib.nix { inherit lib; };
-
-  build =
-    let
   result = lib''.evalModules {
     modules = [
       ./structure
@@ -32,8 +29,8 @@ in
       }
     ];
   };
-    in
-    result.config.build;
+
+  inherit (result.config) build;
 
   shell = pkgs.mkShellNoCC {
     packages = with pkgs; [