Debian needs the libvirt-daemon-system package for the libvirt group
Running check-prerequisites.sh on Debian, I got the message
You need to be in the libvirt group to run Gitian.
Please run:
sudo adduser david libvirt
newgrp libvirt
However the libvirt group didn't exist. The package libvirt-daemon-system is the one that creates it. I suppose it should be added to the list of required packages.