upgrades: backports not available on new stable systems

On stable system with FreedomBox 22.22.1~bpo11+1:

• Matrix cannot be installed.
• Shaarli cannot be installed.
• Software Update page shows "Frequent feature updates are activated."
• /etc/apt/sources.list.d/ is empty.

I noticed this issue in two places:

• bullseye vagrant box with FreedomBox 22.22.1~bpo11+1 installed
• stable container

However on my normal FreedomBox, /etc/apt/sources.list.d/freedombox2.list exists as expected. So I guess the issue only affects newly created systems, where this file should be written during first_boot step if selected.

Unfortunately I did not notice the issue until after 22.22.1~bpo11+1 was uploaded. I am thinking it would be good to have a ConfigFile component, which would enable a diagnostic that checks that a file is present, given some conditions.