It looks like we used to be able to just put the attribute path to nix-env and it worked, like nix-env -iA clang-tools_16, but now you need to add nixpkgs. prefix, so it becomes nix-env -iA nixpkgs.clang-tools_16. When this has changed and why? I guess it is something in nixexprs, but I haven’t looked in it yet.
By the way, nix-env --install - Nix Reference Manual still mentions the old way, so commands from there does not work.