Don't execute job if we are shutting down

[adam-fowler]

No description provided.

[codecov]

Codecov Report

Merging #107 (2d55983) into main (8d2ddd2) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #107      +/-   ##
==========================================
- Coverage   83.39%   83.36%   -0.03%     
==========================================
  Files          68       68              
  Lines        2915     2916       +1     
==========================================
  Hits         2431     2431              
- Misses        484      485       +1     

Impacted Files	Coverage Δ
Sources/HummingbirdJobs/JobQueueWorker.swift	98.71% <100.00%> (-1.29%)	⬇️

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