Elm: fix build by ignoring the build instructions provided by upstream

Re-apply the build fix from <https://github.com/NixOS/nixpkgs/issues/2089>,
because apparently the underlying issue has not been fixed.

According to <https://github.com/elm-lang/Elm/issues/384>, Elm's release
archive comes with a Setup.hs that cannot compile an Elm release. Duh!
Replacing the custom Setup.hs file with a dummy version fixes this issue.