Update fuzz README to account for additional required RUSTFLAGS

[tnull]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.66%. Comparing base (8c086c7) to head (d13c2bd).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3401      +/-   ##
==========================================
- Coverage   89.66%   89.66%   -0.01%     
==========================================
  Files         128      128              
  Lines      104887   104887              
  Branches   104887   104887              
==========================================
- Hits        94052    94042      -10     
- Misses       8126     8136      +10     
  Partials     2709     2709              

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