Setup tests with pure-conda setup

[jezdez]

Checklist

• I added a descriptive title
• I searched open reports and couldn't find a duplicate

What happened?

I think using Pixi for the tests of this conda plugin makes testing more complicated, as it adds a whole new layer of complex tooling between this plugin and conda.

I would encourage us to use the usual pattern to use conda's native capabilities to develop conda plugins.

[jezdez]

I realize that this also only tests the plugin on conda-forge, filed #58