Enlistment when using Custom repo's

After a bit of digging through the deeper caverns of the internet and cobbling together information from a few different sources, i’ve come up with a workaround that works on Maas 3.5 at least.
Update the preseed templates on the region controllers to automatically allow the update when “apt-get update” is run, allowing the enlisting / commissioning to work.
(Not sure if the commissioning override is actually needed though.)

Place an “enlist” and a “commissioning” file in /var/snap/maas/current/preseeds with the following content

root@region:/# cat /var/snap/maas/current/preseeds/enlist
{{preseed_data}}
write_files:
  - path: /etc/apt/apt.conf.d/landscape-fix
    content: |
      Acquire::AllowReleaseInfoChange true;
root@region:/# cat /var/snap/maas/current/preseeds/commissioning
{{preseed_data}}
write_files:
  - path: /etc/apt/apt.conf.d/landscape-fix
    content: |
      Acquire::AllowReleaseInfoChange true;

References: