also build architecture specific lxc.tar for amd64 and arm64
- only build lxc-arm64.tar for unstable
- build lxc-amd64.tar for everything, replacing lxc.tar
After pipeline!444 (merged) was merged, this merge request now makes use of the arm64 autopkgtest image that should now be available. I didn't rename the jobs but added only a single job with the arm64 suffix because I don't think more is needed right now and it would thus be a waste of resources to generate more stuff. After this is merged, I'll file another MR for the pipeline project which removes the fallback code in the autopkgtest job and which documents how to use this new functionality for those packages that need it.