Supervisord: how to append to $PATH

Question

I can't seem to figure out how to append to the default path in a supervisord program config.

I can reset the path:

environment=PATH="/home/site/environments/master/bin"

But when I try:

environment=PATH="/home/site/environments/master/bin:$PATH"

I see that supervisord doesn't evaluate $PATH.

Google wasn't a big help on this for some reason, I cannot believe I'm the first person to need this.

Supervisord must have support for this, any idea what it is?