Remove mamba key from GH Actions

[rly]

Motivation

All conda tests are failing. It seems to be because the experimental mamba: * config is no longer supported / is broken in the setup-miniconda github action. In any case, the latest version of conda uses the libmamba solver by default so we should not need to specify mamba: * anymore.

I do not think this needs a changelog entry, but happy to add one. I added one.

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.02%. Comparing base (fbfabeb) to head (a32e4f1).
Report is 27 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1194      +/-   ##
==========================================
+ Coverage   88.84%   89.02%   +0.18%     
==========================================
  Files          45       45              
  Lines        9879     9879              
  Branches     2812     2812              
==========================================
+ Hits         8777     8795      +18     
+ Misses        777      767      -10     
+ Partials      325      317       -8     

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[rly]

Total runtime is about the same here vs when the workflow was last succeeding, so I think this change does not affect the conda solver speed.

[mavaylon1]

That's nice libmamba is the default now.