use micromamba

[d-chambers]

Description

This PR modifies our current github test action to use a new action for managing a conda (mamba) env which is very lightweight and fast. It also handles caching so I have removed the quirky code that did that manually before.

Unfortunately, I had to follow the micromamba action with a manual env init, but I can maybe figure out how to avoid that later.

Checklist

I have (if applicable):

• referenced the GitHub issue this PR closes.
• documented the new feature with docstrings or appropriate doc page.
• included a test. See testing guidelines.
• your name has been added to the contributors page (docs/contributors.md).
• added the "ready_for_review" tag once the PR is ready to be reviewed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.84%. Comparing base (f4f6947) to head (5659666).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #427   +/-   ##
=======================================
  Coverage   99.84%   99.84%           
=======================================
  Files         112      112           
  Lines        9032     9032           
=======================================
  Hits         9018     9018           
  Misses         14       14           

Flag	Coverage Δ
unittests	99.84% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.