fix: Can't use ReadOnlyReport for on-the-fly comparisons

[scott-codecov]

Fixes https://codecov.sentry.io/issues/4476268908/

[codecov-public-qa]

Codecov Report

Merging #193 (208baea) into main (c425373) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #193   +/-   ##
=======================================
  Coverage   95.61%   95.61%           
=======================================
  Files         600      600           
  Lines       15202    15202           
=======================================
  Hits        14536    14536           
  Misses        666      666           

Flag	Coverage Δ
unit	95.61% <100.00%> (ø)
unit-latest-uploader	95.61% <100.00%> (ø)

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Files	Coverage Δ
services/comparison.py	99.33% <100.00%> (ø)