Reduce memory usage for ios image optimization insights

[NicoHinderling]

Fixes one small potential memory leak + halves the # of workers operating in parallel

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.75%. Comparing base (4d2c903) to head (91e8006).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...aunchpad/size/insights/apple/image_optimization.py	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #309      +/-   ##
==========================================
- Coverage   77.77%   77.75%   -0.03%     
==========================================
  Files         142      142              
  Lines       11147    11149       +2     
  Branches     1184     1185       +1     
==========================================
- Hits         8670     8669       -1     
- Misses       2057     2059       +2     
- Partials      420      421       +1     

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