73f18fd42f
This is useful for adding extra functionality or defaults to _every_ nixos evaluation. My use case is overriding behaviour for all nixos tests, for example setting packageOverrides to newer versions and changing some default dependencies/settings. By making this accessible through an environment variable, this can now be fully accomplished externally. No more need to fork nixos/nixpkgs (which becomes a maintenance burden), just use the channel instead and plug in via this envvar.