Gitian has a couple of different virtualization modes, each with their own quirks and issues. It appears that you are using LXC, which in my experience, no longer works.
You could try the same commands with KVM mode (drop
--lxc and don't set
USE_LXC=1) or docker mode (use
--docker instead of
--lxc and set
USE_DOCKER=1 instead of
USE_LXC). I have found that the docker mode is much more reliable since all of the setup for the container is handled by other projects.
For building older releases, you may need to slightly modify gitian-builder to use different apt package sources as Ubuntu moves them to their archival locations.
For building more recent versions of Bitcoin Core, Gitian has been replaced with Guix. But it's still useful for those who are interested in building older versions of the software.