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

[emaste]

Summary

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 produces packages that are much smaller than uncompressed tarballs
with only a small increase in build time.

[emaste]

@cperciva: @ivy wondered if we could make the default to be lower compression (level 1 or whatever the best tradeoff between speed and size is) and have the official release builds set it to some high level. I think that's reasonable in principle, but I want the release artifacts to be reproducible without requiring users to set a variety of build knobs. What do you think about having one top-level knob to set for official release builds that controls the various defaults for things like this?