[GITIAN] Make the setup feature from gitian-build.py create a debian VM

Summary:
The script builds an Ubuntu Bionic base image while we are using Debian
Buster.

Test Plan:

./contrib/gitian-build.py --setup gitianuser 0.21.11

Check a base-buster-amd64 image is created.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6935