Let the buildd variant only install essential, build-essential and apt

This ensures that source packages are not accidentally built in an environment that also has packages of priority:required installed and thus miss out on build dependencies.

It also makes debootstrap's idea of what packages are needed for building source packages the same as other parts of the archive (apt, dose3, dpkg...).

As an exception to that rule, apt is also installed because buildds use schroot as the sbuild backend which requires apt inside the chroot for build dependency installation.

Closes: #837060