haskell-generic-builder.nix: don't add lib64 directories to the search path

Packages that provide 32 and 64-bit libraries in the same store path are rare
and usually require manual overrides anyway. It seems pointless to try and
guess proper settings for them. The effect is that we'll end up with bogus
settings that take more effort to correct that it takes to configure proper
settings in the first place. Point in case: haskell-cuda and it's configuration
for "cudatools".

Fixes https://github.com/NixOS/nixpkgs/issues/6564.