os-specific/netbsd: fix mandoc splicing
os-specific/netbsd: fix mandoc splicing

This expression is not properly spliced, so we need to
manually specify buildPackages so that mandoc will not
be built for the host platform

Fixes cross-compilation of netbsd.* packages that depend
on mandoc in nativeBuildInputs, such as `getent`.

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