Use micromamba for CI builds

[pont-us]

Fixes #785 .

Update xcube's AppVeyor and GitHub workflow configurations to install and use micromamba rather than installing mamba via conda. See #785 for more details.

Checklist:

• Add unit tests and/or doctests in docstrings n/a
• Add docstrings and API docs for any new/modified user-facing classes and functions n/a
• New/modified features documented in docs/source/* n/a
• Changes documented in CHANGES.md
• GitHub CI passes
• AppVeyor CI passes
• Test coverage remains or increases (target 100%)

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.01 ⚠️

Comparison is base (83e7bb0) 93.18% compared to head (acddefb) 93.18%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #812      +/-   ##
==========================================
- Coverage   93.18%   93.18%   -0.01%     
==========================================
  Files         428      428              
  Lines       33922    33922              
==========================================
- Hits        31611    31609       -2     
- Misses       2311     2313       +2     

see 1 file with indirect coverage changes

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