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.