Improvement: No longer download Doxygen for every build

[spenke91]

Closes #2331

Describe your changes here:

With the PR DLR-AMR/t8code-docker-images#5, Doxygen is downloaded and "installed" in the Dockerfile already, so we no longer have to do so for every build.

UPDATE: Seems to work...

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.57%. Comparing base (5c1e9cb) to head (74e31b4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2333   +/-   ##
=======================================
  Coverage   82.57%   82.57%           
=======================================
  Files         116      116           
  Lines       18553    18553           
=======================================
  Hits        15320    15320           
  Misses       3233     3233           

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