Make it possible to use tails::apt without tails::profile::sysadmins
tails::profile::sysadmins is very specific to the Tails infra. For example:
- It depends on other modules, such as rbac and yapgp.
- It assumes a "zen" user exists.
OTOH, tails::apt is very generic: I've been using it on a number of other systems for many years. This enables a bit more collaboration, and more eyes who can spot potential problems, so there's some value in making it possible to use tails::apt without tails::profile::sysadmins. This was the case before 060df2b1, that introduced a hard dependency.
This commit mitigates the consequences of this change, by restricting the hard dependency to the only situation where it's useful, i.e. when unattended upgrades are enabled and we want to extract the sysadmins' email address from tails::profile::sysadmins. In all other cases, it's now possible again to use tails::apt without tails::profile::sysadmins. And on non-Tails systems where I use unattended upgrades, I'll configure this myself without using this class.