After installing the port in a new FreeBSD 12.1 jail, and starting it with the default 'buildbot' user, the buildbot environment contains a HOME pointing to /root instead of /home/buildbot.
Passing --uid to twistd isn't setting the home directory to the correct place.
A more FreeBSD approach to running with the correct user is to set the variable:
buildbot_worker_uid isn't the best way to do it, since that doesn't cause rc.subr to properly su to the correct user and twistd is itself launched as root.
My workaround now is to set: