fix: don't select nutrition image if one has already been selected

[raphael0202]

Following this bug report on Slack: https://openfoodfacts.slack.com/archives/CF0286BRQ/p1718194556411239?thread_ts=1718190525.623749&cid=CF0286BRQ

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 57.32%. Comparing base (ef96219) to head (035a36b).

Files	Patch %	Lines
robotoff/insights/annotate.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1348      +/-   ##
==========================================
- Coverage   57.33%   57.32%   -0.02%     
==========================================
  Files          79       79              
  Lines        7848     7850       +2     
==========================================
  Hits         4500     4500              
- Misses       3348     3350       +2     

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