Use faster mamba solver in github actions

[rly]

Motivation

GitHub Actions sometimes gets stuck solving the conda python 3.12 environment. Use mamba instead of the default conda solver. It is faster and less prone to getting stuck. This is minor enough not to need a changelog IMO.

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running ruff from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a270a14) 88.38% compared to head (00c3667) 88.38%.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #992   +/-   ##
=======================================
  Coverage   88.38%   88.38%           
=======================================
  Files          45       45           
  Lines        9565     9565           
  Branches     2720     2720           
=======================================
  Hits         8454     8454           
  Misses        784      784           
  Partials      327      327           

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[h-mayorquin]

Maybe this is useful, apparently conda latest version allows to use mamba solver:

https://www.anaconda.com/blog/a-faster-conda-for-a-growing-community