generic-builder: use buildInputs for Haskell dependencies

This actually will matter when I (soon) land cross-compilation support,
and native deps are compiled for the build, instead of host, platform.
But even now, it's good style do this, and one less thing to review
later.

The use of `$nativePkgs` is unfortunate, as it will need to be
swapped out for `$crossPkgs` for build != host builds. This will just a
temporarily cause pain, however, as eventually I will change stdenv to
use a `$runPkgs` (or moral equivalent) in both case.

Closes https://github.com/NixOS/nixpkgs/pull/26480.