FYI, stolen from vagrant-omnibus plugin:
https://github.com/schisamo/vagrant-omnibus/blob/master/lib/vagrant-omnibus/action/install_chef.rb#L29-L30

Needed so that `env[:global_config].librarian_chef.cheffile_dir` below would be finalized as `nil` in the `call` method, rather than remaining as a funky vagrant `UNSET_VALUE` object