Using Mamba instead of Conda

[mamba](https://github.com/thesnakepit/mamba) is a dropin replacement for the conda package manager, which uses the same archive format as conda. Being written in c++, it is much much faster than conda itself, which matters a lot to me given the number of time, I happen to be recreating Julia/Python environement. Given it takes the same options as conda, I suppose it could be swapped or used optionally in Conda.jl with relatively little effort.

There is also an alternative, micromamba, which might also be useful in the Julia context, since it can be used to bootstrap a conda environment with no dependence on a Python executable (for instance, to install R libraries).