Supervisord: how to append to $PATH

This feature has been added to Supervisor back in 2014

environment=PATH="/home/site/environments/master/bin:%(ENV_PATH)s"

see https://github.com/Supervisor/supervisor/blob/95ca0bb6aec582885453899872c60b4174ccbd58/supervisor/skel/sample.conf#L7

See also https://stackoverflow.com/questions/12900402/supervisor-and-environment-variables