mmdebstrap-created images are incompatible with autopkgtest's setup-testbed

The mmdebstrap Debusine task produces an environment that autopkgtest's setup-testbed cannot deal with automatically. The symptom is:

awk: cannot open "/etc/apt/sources.list" (No such file or directory)
Failed to auto-detect distribution release name; set $RELEASE explicitly
E: setup failed: E: command failed: chroot "$1" /customization_script