stack: prefer the ghc-8.6.1 version

Building stack with ghc-8.4.x requires a whole lot of version overrides -- most
notably Cabal 2.4.x --, so we don't bother and just use the one built with the
latest compiler instead.

pkgs/development/haskell-modules/configuration-ghc-8.4.x.nix

@@ -56,4 +56,8 @@ self: super: {
           };
     in appendPatch super.hadoop-rpc patch;
 
+  # Version 1.9.1 needs Cabal 2.4.x or later, so
+  # we use the one from the ghc-8.6.1 package set.
+  stack = markBroken super.stack;
+
 }