Is specifying nixpkgs necessary to build sd-image?

currently I am using such a script to build an sd image using a nixpkgs version pinned in sources.json is there something i can simplify about this? sources="$(pwd)/sources.json" nixpkgs=$(nix-instantiate --eval --expr "(builtins.fetchTarball (builtins.fromJSON (builtins.readFile $sources)).nixpkgs)"| tr -d \") nix-build '<nixpkgs/nixos>' -A config.system.build.sdImage --argstr system aarch64-linux -I nixos-config=./sd-image.nix -I nixpkgs=$nixpkgs