laravel envoy - how can I choose between different deploy paths?

I have two servers, /var/www/html and /srv/www/html

I want to be able to select the deploy path depending on the $on variable, but when I try to capture it, whether in @setup or @macro it's NULL.

There's nothing in the manual, and all the online tutorials I look at stop short of dealing with it. Where and how can I capture $on to set the deploy path?

via Chebli Mohamed