support env vars in group definitions

Now that names can contain env vars, group definitions should also be expanded, otherwise it's impossible to refer to containers with a "dynamic name".  I can't think of any use-case right now for having env vars in group names themselves, so I did not push as far as refactoring `RawGroups`/`Groups()`.

Anyway, thanks for the now-systematic env var expansion for run parameters, very useful!