xmonad: Don't add ghc to the closure when 'config' is set

When 'services.xserver.windowManager.xmonad.config' is not null, the
restart feature won't work by default and is not desirable unless
specific care is made, as explained by the documentation.

I think it's reasonable to not include the haskell dependencies in the
environment by default. That reduces the size of my system by 2GB.

nixos/modules/services/x11/window-managers/xmonad.nix

@@ -29,7 +29,6 @@ let
       } ''
         install -D ${xmonadEnv}/share/man/man1/xmonad.1.gz $out/share/man/man1/xmonad.1.gz
         makeWrapper ${configured}/bin/xmonad $out/bin/xmonad \
-          --set NIX_GHC "${xmonadEnv}/bin/ghc" \
           --set XMONAD_XMESSAGE "${pkgs.xorg.xmessage}/bin/xmessage"
       '';
 
@@ -93,6 +92,8 @@ in {
           <literal>(restart "xmonad" True)</literal> instead, which will just restart
           xmonad from PATH. This allows e.g. switching to the new xmonad binary
           after rebuilding your system with nixos-rebuild.
+          For the same reason, ghc is not added to the environment when this
+          option is set.
 
           If you actually want to run xmonad with a config specified here, but
           also be able to recompile and restart it from a copy of that source in