[CI/code quality] Add clear github runner caches job + mypy fixes

[felixdittrich92]

This PR:

• Add a small CI job to clear github runner caches which are older than 2 days
• Fix mypy issues

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.53%. Comparing base (99842ba) to head (1350aaf).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1783      +/-   ##
==========================================
- Coverage   96.58%   96.53%   -0.06%     
==========================================
  Files         165      165              
  Lines        7901     7901              
==========================================
- Hits         7631     7627       -4     
- Misses        270      274       +4     

Flag	Coverage Δ
unittests	96.53% <100.00%> (-0.06%)	⬇️

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