Don't ship the testing and unstable APT sources
I.e. only have them enabled while building Tails — after that they’re
arguably of little use since users should install stable packages. This
would significantly speed up `apt update` — right now we have to fetch
almost 40 MiB, but after removing testing
and unstable
only 13.5 MiB
is fetched, so a ~200% speed-up can be expected.
Related issues
- Related to #12237 (closed)
Original created by @anonym on 11539 (Redmine)