You haven’t told where lib
should come from…
Perhaps the following might work:
nix-build '<nixpkgs/nixos>' - <<'EOF'
let pkgs = import <nixpkgs> {}; in
(import <nixpkgs/nixos/lib/make-disk-image.nix> {
inherit (pkgs) lib config;
pkgs = import <nixpkgs> { inherit (pkgs) system; };
diskSize = 10240;
format = "qcow2";
}).system.build.qcow2
EOF