explicitly specify the micromamba version

[finozzifa]

Closes # (if applicable).

Fix

  /home/runner/micromamba-bin/micromamba list -r /home/runner/micromamba -n pypsa-earth
  critical libmamba Error parsing MatchSpec "pypsa[version='>=0.24,',build='<0.25']": Error setting attribute "version" to value ">=0.24,": Version spec "" is an incorrect expression"
  List of packages in environment: "/home/runner/micromamba/envs/pypsa-earth"

As explained here, setting the micromamba version explicitly fixes the problem.

Changes proposed in this Pull Request

Checklist

• I consent to the release of this PR's code under the AGPLv3 license and non-code contributions under CC0-1.0 and CC-BY-4.0.
• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Newly introduced dependencies are added to envs/environment.yaml and doc/requirements.txt.
• Changes in configuration options are added in all of config.default.yaml and config.tutorial.yaml.
• Add a test config or line additions to test/ (note tests are changing the config.tutorial.yaml)
• Changes in configuration options are also documented in doc/configtables/*.csv and line references are adjusted in doc/configuration.rst and doc/tutorial.rst.
• A note for the release notes doc/release_notes.rst is amended in the format of previous release notes, including reference to the requested PR.

[davide-f]

Thanks @finozzifa :D