ci: bump Ubuntu to 20.04
ci: bump Ubuntu to 20.04

This also removes the manual addition of the universe repository, since it's already enabled in the circleci image.
Because of this, software-properties-common doesn't need to be installed anymore, and libnss-systemd isn't going to be installed (see c4257802206186b891bea8d055c14b03772a64bf).
However, DEBIAN_FRONTEND=noninteractive needs to be set, else the image will loop while trying to configure tzdata (aka the timezome).

Signed-off-by: Stephan Lachnit <02f37ba4c3a5bdeb31cf202876802e9c254d5236@protonmail.com>

libratbag/libratbag,whot/libratbag,whot/libratbag,whot/libratbag,libratbag/libratbag,libratbag/libratbag