Use plain miniconda in CI

[sfinkens]

Do not use mamba at all. From slack discussion:

I just tested and yeah not using mamba works as well as forcing the mamba version

• Closes Unpin mamba version in CI #2917

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.06%. Comparing base (6a4f2af) to head (9fb59bd).
Report is 16 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2919   +/-   ##
=======================================
  Coverage   96.06%   96.06%           
=======================================
  Files         373      373           
  Lines       54406    54406           
=======================================
  Hits        52267    52267           
  Misses       2139     2139           

Flag	Coverage Δ
behaviourtests	3.99% <ø> (ø)
unittests	96.16% <ø> (ø)

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[coveralls]

Pull Request Test Coverage Report for Build 11138990341

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.164%

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Totals
Change from base Build 11128129441:	0.0%
Covered Lines:	52501
Relevant Lines:	54595

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💛 - Coveralls