CI: Rely on Debian job definitions as provided by the Debian CI team
These configs and job definitions are team maintained, which is preferable, as up until now, our configs were only handled by a single person.
Besides this, explicitly fetch the Debian branch before the Debian build. Not doing so lead to fatal git errors in some cases like "[...] not something we can merge", due to the git clone and fetch limits as introduced in ee182d7a.