Use micromamba instead of miniconda to speed up tests

[haasad]

If we use micromamba instead of miniconda to run the tests in the github actions, we can bring down the time from ~20min to ~6min. This is still far from ideal, but at least the initial feedback is much faster with linux tests only taking ~2min.

relates: #1094 + #1097

Checklist

• Keep pull requests small so they can be easily reviewed.
• Categorize the PR by setting a good title and adding one of the labels:
  bug, feature, ui, change, documentation, breaking, ci
  as they show up in the changelog
• Link this PR to related issues by using closing keywords.
• Request a review from another developer.

[coveralls]

coverage: 50.0% (-0.07%) from 50.066%
when pulling de49deb on micromamba-actions
into 7eebd42 on master.

[github-actions]

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