The binary will be in `/run/wrappers/bin` and adding `pkgs.iotop` won't have any effect. See also https://github.com/NixOS/nixpkgs/pull/51749#discussion_r254724170
*** NixOS *** NixOS is a Linux distribution based on the purely functional package management system Nix. More information can be found at http://nixos.org/nixos and in the manual in doc/manual.