configs/releng/profiledef.sh: use zstd compression for bootstrap image
The zstd tool has now been around for a while, so the availability of it should not be a concern anymore.
Unlike gzip which was used until now, zstd offers higher compression
while still being faster (and multi-threaded).
The --auto-threads=logical option is used just so that there is some
difference between the releng and baseline profiles.
Everyone using the official Arch Linux bootstrap tarball (previously
archlinux-bootstrap-YYYY.MM.DD-x86_64.tar.gz or
archlinux-bootstrap-x86_64.tar.gz) will need to update their scripts
and etc. to use archlinux-bootstrap-YYYY.MM.DD-x86_64.tar.zst or
archlinux-bootstrap-x86_64.tar.zst instead.
Implements #130 (closed)