Fixes:
Warning: `stdenv.lib` is deprecated and will be removed in the next release. Please use `pkgs.lib` instead. For more information see https://github.com/NixOS/nixpkgs/issues/108938
The newer versions of ghcjs use emscripten to deal with .c code.
The version of emscripten that is available by default in nixpkgs
does not include the "upstream" version of emscripten.
This commit takes the existing emscripten from nixpkgs and adds
the "upstream" version.