Avoid calling hook script multiple times

[Martchus]

• Call the hook script only if the job was not already in state DONE.
• Note that the "done" function might be called multiple times, e.g. by the worker and also by the stale job detection.
• There is still a small possibility that the hook script is called multiple times but it is hopefully not relevant in practice.
• See https://progress.opensuse.org/issues/169747

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.98%. Comparing base (d360bdf) to head (d7e0ff6).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6068   +/-   ##
=======================================
  Coverage   98.98%   98.98%           
=======================================
  Files         395      395           
  Lines       39492    39508   +16     
=======================================
+ Hits        39092    39108   +16     
  Misses        400      400           

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