Improve efficiency of github actions workflows

[gassmoeller]

This change improves the efficiency of the world builder testers by taking over the following changes from workflows of other CIG software:

1. Only execute tests when pushing to the main branch (=merging) or pushing to an open pull request. Do not execute when pushing to private branches on github forks.
2. Cancel in progress tester runs if a new commit is pushed to a pull request. Since a new run has to be done anyway, there is no point in finishing running tests.

Since WB testers are fast there is not much gained in terms of development speed, but this will save some energy somewhere in the cloud.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (05df5d4) 93.55% compared to head (bc0e464) 93.55%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #561   +/-   ##
=======================================
  Coverage   93.55%   93.55%           
=======================================
  Files          92       92           
  Lines        6342     6342           
=======================================
  Hits         5933     5933           
  Misses        409      409           

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[MFraters]

I agree that this is useful.