Switch CI to micromamba

[jbusecke]

Same as https://github.com/xgcm/xgcm/pull/577/files

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (6a60c3c) 90.03% compared to head (1f66669) 90.03%.

❗ Current head 1f66669 differs from pull request most recent head af4f939. Consider uploading reports for the commit af4f939 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #61   +/-   ##
=======================================
  Coverage   90.03%   90.03%           
=======================================
  Files           7        7           
  Lines         251      251           
=======================================
  Hits          226      226           
  Misses         25       25           

Flag	Coverage Δ
unittests	90.03% <ø> (ø)

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