Use conda's libmamba solver

[jakirkham]

Now that conda-libmamba-solver ships with the installer. Opt-in to using the libmamba solver with conda.

[jakirkham]

We can make this change here, which may be valuable just to have in the Docker images

Though we can also make this change in conda-forge-ci-setup to cover all OSes: conda-forge/conda-forge-ci-setup-feedstock#259

Or we can do both

[jaimergp]

I'd say we stick to conda-forge/conda-forge-ci-setup-feedstock#259. The default will soon change anyway.

[jakirkham]

Yeah guess the benefit of having it here also would be the Docker image is preconfigured (as opposing to adding a step) for users that just grab the Docker image. Though have no strong feelings (given this is an easy change to make and it soon will be the default anyways)

[jaimergp]

I think I'll close here in favor of conda-forge/conda-forge-ci-setup-feedstock#275, to minimize moving parts. As you say, it will be default soon anyways, so if it breaks for non-conda-forge usage, it will be because of that, not because we rolled the change early.