customization_script in mmdebstrap task cannot use network

When providing a customization_script to a mmdebstrap task, the script cannot use network, because /etc/resolv.conf is deleted from the chroot right before invoking the script and hence DNS resolution fails.

While relevant packages can be included via an option, the main way to execute the autopkgtest customization script is to apt-get download autopkgtest (see also #1043 (closed)).