backports: work nicer with Raspbian / Ubuntu

A user on IRC reported an error after installing the freedombox package on Raspbian:

Err:1 http://deb.debian.org/debian buster-backports InRelease<br />
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 04EE7237B7D453EC NO_PUBKEY 648ACFD622F3D138
Get:4 http://archive.raspberrypi.org/debian buster/main armhf Packages [234 kB]
Reading package lists... Done<br />
W: GPG error: http://deb.debian.org/debian buster-backports InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 04EE7237B7D453EC NO_PUBKEY 648ACFD622F3D138

We should skip enabling backports in this case. If the user really wants backports, they will need to add the Debian repository keys manually.

EDIT: A user on the Matrix channel mentioned that they ran into the same problem, but are using Ubuntu on a RasPi4.