nixos: allow adding extra modules through environment

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.
BTW, not sure if it's a good idea to have this as a function argument, because that allows a caller to `eval-config.nix` to turn off the user's extra modules, which seems to negate the whole point...