Allow conda solver configuration, with libmamba as default

[jaimergp]

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

Supersedes #259

Documentation at conda-forge/conda-forge.github.io#2003

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[jaimergp]

This should be the last PR to be merged in the conda-forge/conda-forge.github.io#2006 collection, since it will have immediate effect. I'd like to have the conda-smithy changes released before too so we are not mixing conda-libmamba-solver and mamba in the same CI.

[jaimergp]

I think I am going to move this to conda-smithy. If we do it conda-forge-ci-setup, we have missed to conda install|update instances where the solver was not configured as intended (would use the default value in conda).

[jaimergp]

Absorbed in conda-forge/conda-smithy#1768