haskell generic-builder: fix Darwin regression for lmdb

Recent updates to the generic builder have caused haskellPackages.lmdb-simple to
fail to build on Darwin, since it cannot see the lmdb C dynamic library included
by its dependent haskellPackages.lmdb.

The C dynamic library has suffix `.so` not `.dylib`, so this fix allows for
that.

Closes #80190, but that issue may identify a preferable solution.