fix(worker): Run export/fsck tasks with J4

[nellh]

Temporarily increasing the concurrency to 4 jobs per export or fsck.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.01%. Comparing base (24b4287) to head (abed58c).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3657      +/-   ##
==========================================
- Coverage   43.02%   43.01%   -0.01%     
==========================================
  Files         636      636              
  Lines       33888    33888              
  Branches     1515     1515              
==========================================
- Hits        14579    14578       -1     
- Misses      19170    19171       +1     
  Partials      139      139              

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