ci: update to match apertis-flatdeb-demo

Similarly to what has been done in apertis-flatdeb-demo, the CI pipeline
has been extended to move repository publishing away from the build
stages. It now builds the repos for all supported architectures, while
automatically generating the required files based on the branch name.

Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>