Many nix-env commands work by trying to evaluate every package in Nixpkgs so if there are any errors or warnings, they will be printed.
Looks like the error is probably fallout from mesa: split out the Darwin build into a separate expression by K900 · Pull Request #319047 · NixOS/nixpkgs · GitHub.
CI intentionally builds with aliases disabled to detect alias usage, which is forbidden in Nixpkgs. So that’s probably why it was not caught.
You can disable allowAliases in Nixpkgs configuration (in ~/.config/nixpkgs/config.nix).