Cirrus-CI: Switch back to tzst packages, but use level 1

We previously used PKG_FORMAT: tar to avoid spending a lot of time in
zstd compression. Instead just set PKG_LEVEL to compression level 1,
which still produces packages that are much smaller than uncompressed
tarballs with only a small penalty in build time.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52858