Use micromamba instead of miniconda

[lmmx]

According to the tox-conda README:

If mamba is installed in the same environment as tox, you may use it instead of the conda executable by setting the environment variable CONDA_EXE=mamba in the shell where tox is called.

There is a version of mamba that’s completely statically linked called ‘micromamba’ which is only 5MB, which would greatly speed up tox-conda CI.

• See micromamba docs
• Setting CONDA_EXE=micromamba would allow this to be used with tox-conda