add support for arm64-only pipelines

Hi,

while my package box64 builds on amd64, it is useless on that architecture because box64 is an emulator for amd64. Thus, I have disabled amd64 in my d/control. Unfortunately, there is not yet support to run all relevant pipeline steps for a given package for an architecture other than amd64. With help from @santiago I prepared this merge request which adds support for this use-case. It contains two commits:

1. build images for autopkgtest, blhc, piuparts and reprotest based on arm64
2. prepare for a change in autopkgtest-lxc which will provide an architecture specific lxc.tar -- to that end, the current implementation contains a fall-back to the old lxc.tar so that during the time that autopkgtest-lxc is not changed, nothing will break.

An example of how salsaci.yml which uses arm64 for the relevant jobs can look like can be found here: https://salsa.debian.org/debian/box64/-/blob/master/debian/salsa-ci.yml