Create zsync control file for delta downloads

A zync control file is a small file (~1.5MB) containing hashes of blocks that can be used by zsync to download a delta based on a previously downloaded file. This approach works on any HTTP server that supports download ranges. In the case of archiso it can reduce data download by 40-50%, and will use any older (or indeed newer) image as a seed file with no additional effort required:

zsync2 -i archlinux-2020.12.01-x86_64.iso https://gitlab.archlinux.org/jonathon/archiso/-/jobs/16578/artifacts/raw/output/releng/archlinux-2021.03.04-x86_64.iso.zsync

This MR adds zsync control file creation to the CI build process at the same stage as creating checksums.