maint: Handle environment vars in micromamba/tests with monkeypatch

[holzman]

Description

Addresses #3830.

Nearly all of places where environment variables were set in the tests were needed, so I didn't change very much.

Type of Change

• Bugfix
• Feature / enhancement
• CI / Documentation
• Maintenance

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.57%. Comparing base (7cdf568) to head (d6d74d8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3988   +/-   ##
=======================================
  Coverage   63.57%   63.57%           
=======================================
  Files         300      300           
  Lines       38401    38401           
  Branches     2861     2861           
=======================================
  Hits        24412    24412           
  Misses      13922    13922           
  Partials       67       67           

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[holzman]

Weird, I must have broken something that worked for me locally. I'll take a look next week.

[holzman]

Oh, not my fault!

test_update.py::TestUpdateConfig::test_target_prefix_with_no_settings should have always been failing. It was only succeeding because MAMBA_ROOT_PREFIX was altered by earlier tests as a side effect.