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

[mooori]

Before the justfile would just use target/release/neard, which can be unexpected. Changes this and adds docs explaining how to set the binary used in benchmarks.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.40%. Comparing base (c625d98) to head (f3a642b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12817      +/-   ##
==========================================
- Coverage   70.41%   70.40%   -0.01%     
==========================================
  Files         848      848              
  Lines      174930   174930              
  Branches   174930   174930              
==========================================
- Hits       123169   123166       -3     
- Misses      46506    46509       +3     
  Partials     5255     5255              

Flag	Coverage Δ
backward-compatibility	0.16% <ø> (ø)
db-migration	0.16% <ø> (ø)
genesis-check	1.41% <ø> (ø)
linux	70.02% <ø> (+<0.01%)	⬆️
linux-nightly	70.05% <ø> (ø)
pytests	1.70% <ø> (ø)
sanity-checks	1.52% <ø> (ø)
unittests	70.24% <ø> (-0.01%)	⬇️
upgradability	0.20% <ø> (ø)

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[ssavenko-near]

one minor comment. Otherwise LGTM