remove mamba & conda mixture

[jGaboardi]

The current rtree failures may be due to my mixing of conda & mamba in CI. Although I had set channel-priority: true, it was being implemented as flexible in the workflow.

See xref

@martinfleis

[martinfleis]

So the issue was a mixture of channels in the end? If so, we can close the geopandas issue with that conclusion.

[jGaboardi]

So the issue was a mixture of channels in the end? If so, we can close the geopandas issue with that conclusion.

@martinfleis I thought so... but comparing the log the failed to the log the passed both have:

channel_priority: flexible
channels:
  - conda-forge
  - defaults

I am thoroughly confused now...