Switch CI to use micromamba

[mcflugen]

Description

This pull request switches our CI for testing from miniconda to micromamba. It's probably something we should have already been doing but recent build errors seem to be fixed by the switch.

The errors we were seeing were things like,

Traceback (most recent call last):
  File "C:\Miniconda\envs\test\Scripts\mamba-script.py", line 6, in <module>
    from mamba.mamba import main
  File "C:\Miniconda\envs\test\Lib\site-packages\mamba\mamba.py", line 49, in <module>
    import libmambapy as api
  File "C:\Miniconda\envs\test\Lib\site-packages\libmambapy\__init__.py", line 7, in <module>
    raise e
  File "C:\Miniconda\envs\test\Lib\site-packages\libmambapy\__init__.py", line 4, in <module>
    from libmambapy.bindings import *  # noqa: F401,F403
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ImportError: DLL load failed while importing bindings: The specified module could not be found.

Checklist - did you ...

• Add a news fragment file entry if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?
• All tests have passed?
• Formatted code with black?
• Removed lint reported by flake8?
• Sucessful documentation built? (if documentation added or modified)