make-derivation: allow nested lists in buildInputs
make-derivation: allow nested lists in buildInputs

This isn't really desirable in general, but given that Nix itself
currently relies on this behaviour and that we don't want to break
backwards compatibility we should support it for now, maybe
deprecating it in the future.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs