generic Haskell builder: Don't call stack setup.

`stack setup` should not be called inside `nix-build` because
that makes stack download a GHC instead of using nix's one
(the one provided `generic-stack-builder.nix` as the `ghc` argument),
which will not work.
Further evidence for this is that the `stack --nix` support
intentionally uses `--system-ghc` for the same purpose.

pkgs/development/haskell-modules/generic-stack-builder.nix

@@ -37,7 +37,6 @@ stdenv.mkDerivation (args // {
 
   configurePhase = args.configurePhase or ''
     export STACK_ROOT=$NIX_BUILD_TOP/.stack
-    stack setup
   '';
 
   buildPhase = args.buildPhase or "stack build";