pkgs-lib: fix JSON, YAML and TOML cross-compilation
pkgs-lib: fix JSON, YAML and TOML cross-compilation

Splicing of nativeBuildInputs doesn't work unless callPackage is used, so
the generators were attempting to use host platform tools at build time.

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