Since past week I have noticed the install link displayed on https://search.nixos.org/packages?query=nomachine&from=0&size=15&sort=relevance&channel=unstable displays
nix-env -iA nixos.nomachine-client rather than
nix-env -iA nixpkgs.nomachine-client
the install works after I change the link to
nix-env -iA nixpkgs.nomachine-client though. However I have noticed that by default unstable channel used to display nixpkgs link and not nixos. I suppose something has changed if this is not an error and an intended behaviour.