I'm also not a nix expert and I'm not too familiar with Haskell build ecosystem, but doesn't the buildTarget argument do what you need?
https://nixos.org/manual/nixpkgs/stable/#haskell-derivation-args (it's the last on the list)
Name of the executable or library to build and install. If unset, all available targets are built and installed.
I created an account just for that lol