haskell infra: Make sure packages get custom stdenv

One of the goals of 74f5fe5 was to allow passing in a custom stdenv,
which would be used for genericBuilder's `mkDerivation` call. That does
work, but if packages takes `stdenv` as an parameter for any reason,
they'll get the default one instead. This change remedies it.

pkgs/development/haskell-modules/make-package-set.nix

@@ -89,7 +89,7 @@ self: let
       };
     in stdenv.lib.makeOverridable drvScope (auto // manualArgs);
 
-  mkScope = scope: pkgs // pkgs.xorg // pkgs.gnome2 // scope;
+  mkScope = scope: pkgs // pkgs.xorg // pkgs.gnome2 // { inherit stdenv; } // scope;
   defaultScope = mkScope self;
   callPackage = drv: args: callPackageWithScope defaultScope drv args;