Fix race condition on task retry

[RB387]

Related issue:
#482

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@@            Coverage Diff             @@
##             main     #487      +/-   ##
==========================================
- Coverage   96.27%   96.10%   -0.18%     
==========================================
  Files          11       11              
  Lines        1074     1078       +4     
  Branches      209      190      -19     
==========================================
+ Hits         1034     1036       +2     
- Misses         19       21       +2     
  Partials       21       21              

Files with missing lines	Coverage Δ
arq/worker.py	97.37% <100.00%> (+0.20%)	⬆️

... and 1 file with indirect coverage changes

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[RB387]

@samuelcolvin @chrisguidry 🙏

[chrisguidry]

This makes sense to me, thanks @RB387!