Add .mambarc to speed up index download

[corneliusroemer]

Description of proposed changes

We download package indexes a lot. A few months ago, micromamba started supporting the download of zstd compressed files instead of gzip. Downloading an index can be sped up by ~10s.

Enabling is as simple as adding repodata_use_zst in the .mambarc (or adding an appropriate env variable). If zst file is not available, there's a fallback, so this shouldn't break anything.

In my tests, this seems to reduce build times by ~20%.

Compare:

• with zst (this PR): https://github.com/nextstrain/conda-base/actions/runs/4981847694
• without zst (status quo): https://github.com/nextstrain/conda-base/actions/runs/4981414616

If this works well, we could try to add this env variable to any workflow that sets up micromamba.

[tsibley]

Can we just wait until this is the default? The option isn't even documented yet… and it seems like the intent of the option is to hide this experimental behaviour until it (soon?) becomes the default.

Can this go in the existing .condarc instead of a new .mambarc? I'd think it could.

In my tests, this seems to reduce build times by ~20%.

FWIW, I see ~11% total time improvement between the two linked CI runs, with individual "build and test" job improvement of ~16% for the Linux build and ~19% for the macOS build.

[victorlin]

Closing since libmamba now uses ZST as of version 1.4.7.