Prefer mamba and conda forge channels

Created by: Zaharid

The whole issue with #1279 (closed) brings to attention that conda forge now has a lot of public documentation, one can see who maintains the recipes and probably so many variations that something like #1279 (closed) would be more apparent.

On top of that there is the mamba project which solves dependencies a lot faster than conda (which can be borderline unusable). With that I would suggest to update https://github.com/NNPDF/binary-bootstrap/blob/master/bootstrap.sh as well as the CI code and relevant pieces of the documentation to have conda forge as higher priorities than defaults and use mamba instead of conda.