Fixes #2379. The new name was a misnomer because the values really are X11 video drivers (e.g. ‘cirrus’ or ‘nvidia’), not OpenGL implementations. That it's also used to set an OpenGL implementation for kmscon is just confusing overloading.
Nixpkgs is a collection of packages for Nix package manager. Nixpkgs also includes NixOS linux distribution source code.