bench(synth-bm): make binary selection more explicit

[mooori]

Replaces #12817 where resolving merge conflicts has gone wrong.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.54%. Comparing base (0e21730) to head (bd09233).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12897      +/-   ##
==========================================
- Coverage   70.54%   70.54%   -0.01%     
==========================================
  Files         851      851              
  Lines      174914   174914              
  Branches   174914   174914              
==========================================
- Hits       123395   123385      -10     
- Misses      46397    46405       +8     
- Partials     5122     5124       +2     

Flag	Coverage Δ
backward-compatibility	0.36% <ø> (ø)
db-migration	0.36% <ø> (ø)
genesis-check	1.42% <ø> (ø)
linux	70.40% <ø> (+<0.01%)	⬆️
linux-nightly	70.16% <ø> (-0.03%)	⬇️
pytests	1.73% <ø> (ø)
sanity-checks	1.54% <ø> (ø)
unittests	70.37% <ø> (-0.01%)	⬇️
upgradability	0.36% <ø> (ø)

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