Packer seems to overwrite /etc/apt/sources.list.
Add the following to packer-maas/ubuntu/user-data-<whatever>:
apt:
primary:
- arches: [default]
uri: <URL for sources.list>
Then change the file around with sed if you need to change things:
- sed -i '/lunar-updates/d' etc/apt/sources.list
- sed -i '/lunar-backports/d' etc/apt/sources.list
- sed -i '/lunar-security/d' etc/apt/sources.list