Explicitly install apt-transport-https and ca-certificates

Raspbian buster is old enough that this isn't included by default, but it looks like the mirror redirector now started redirecting to HTTPS mirrors. Thus, install it so this has a chance to actually work.

The previous job for branch buster did not install these packages: https://git.icinga.com/build-docker/raspbian-base/-/jobs/150159#L235

The pipeline for this MR does: https://git.icinga.com/build-docker/raspbian-base/-/jobs/150163#L244 https://git.icinga.com/build-docker/raspbian-base/-/jobs/150163#L254