bundlerEnv: Fix passthru not being passed-through

E.g. `pkgs.bundlerEnv { passthru = { ... }; ... }` previously would not
result in the `passthru` attrs not being concatenated onto the returned
derivation. Now it will be.

pkgs/development/ruby-modules/bundled-common/default.nix

@@ -23,6 +23,7 @@
 , nativeBuildInputs ? []
 , buildInputs ? []
 , extraConfigPaths ? []
+, passthru ? {}
 , ...
 }@args:
 
@@ -182,7 +183,7 @@ let
             exit 1
           '';
         };
-    });
+    } // passthru);
   };
 
   basicEnv =