Use the system `libffi` (`ie` nixpkgs's) instead of built-in libffi
from ghc source tree.
This will prevent library conflict when ghc dynamically links haskell
packages (linked with ghc built-in libffi) and any external library
which uses nixpkgs `libffi`.
Closes https://github.com/NixOS/nixpkgs/pull/55208.