Pin micromamba version in CI

[brynpickering]

Fixes failing tests seen in #690

This is caused by latest micromamba updating to v2.0 and lots of issues arising. This one seems to have been flagged by someone else here

Summary of changes in this pull request

• Pin to most recent v1 micromamba

Reviewer checklist

• Test(s) added to cover contribution
• Documentation updated
• Changelog updated
• Coverage maintained or improved

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.98%. Comparing base (683a5b7) to head (dd6fef2).
Report is 21 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #691   +/-   ##
=======================================
  Coverage   95.98%   95.98%           
=======================================
  Files          26       26           
  Lines        3981     3981           
  Branches      837      768   -69     
=======================================
  Hits         3821     3821           
  Misses         70       70           
  Partials       90       90           

[irm-codebase]

This looks good overall.
However, I would open an issue to keep track of this, as we most likely want to update mamba sooner rather than later.