[CI] Use micromamba instead of miniforge3

[pitrou]

Describe the enhancement requested

There is a standalone no-dependencies binary installation of mamba called micromamba:
https://mamba.readthedocs.io/en/latest/installation/micromamba-installation.html

We could probably use it instead of our current miniforge install.

Of course, this is a minor improvement. It might make the setup step slightly faster, and would probably make the CI code slightly simpler.

Component(s)

Continuous Integration

[jjerphan]

We recommend using micromamba's dedicated GitHub Action, mamba-org/setup-micromamba.

[pitrou]

Ah, but we're installing it in a Docker image.

[jjerphan]

In this case, mamba-org/micromamba-docker might be of an inspiration.

[pitrou]

Apparently there are docs for it: https://micromamba-docker.readthedocs.io/en/latest/