Quiet conda warnings.

[bdice]

This fixes failures in the new testing workflow from #690.

Update: the root cause was that rapids-conda-retry is sending 2>&1. The warning from conda is being sent to stderr as intended, but we're getting it in stdout because of rapids-conda-retry behavior. We can still solve this by providing --quiet, without needing to change rapids-conda-retry. The old issue contents below are partially incorrect so I collapsed them:

Old issue contents

Output like this is shown, even with the --json flag to conda:

==> WARNING: A newer version of conda exists. <==
    current version: 24.9.0
    latest version: 24.9.1

Please update conda by running

    $ conda update -n base -c conda-forge conda

The only way to make the output "proper JSON" is to pass --quiet as well.

This seems like unintentional behavior from conda. The docs from conda create --help literally say:

--json                Report all output as json. Suitable for using conda programmatically.

[msarahan]

Annoying, though I must admit I've probably introduced this same kind of error myself over the years.

[bdice]

/merge