worker: Adjust "Failed to run job" error logging

[Turbo87]

Adding the error details as a structured logging field isn't actually as useful. Let's revert this back to regular stringification.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.72%. Comparing base (087d070) to head (77c6be7).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/crates_io_worker/src/worker.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9687   +/-   ##
=======================================
  Coverage   88.71%   88.72%           
=======================================
  Files         287      287           
  Lines       29338    29336    -2     
=======================================
  Hits        26027    26027           
+ Misses       3311     3309    -2     

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