build.sh: create temporary apt cache
- allows running everything on something that is not Debian unstable or testing
- does not require superuser privileges to update /etc/apt/sources.list
- avoid requiring apt one-line format on the host
- makes sure that the package cache is always up-to-date
I am working on this because I'm on bookworm but I don't think there is any reason to forbid that as the package builds are all inside an sbuild chroot, correct?