[BUILD] Fix benchmark genrule capturing stderr into JSON output

[ThomsonTan]

Fixes #3926

Changes

The otel_cc_benchmark genrule used &> to redirect both stdout and
stderr into the _result.json file. SDK warnings (e.g.,
WarnOnDuplicateInstrument from measurements_benchmark) written to stderr
corrupt the JSON, causing gojq to fail with "invalid json" in CI.

Change &> $@ to > $@ 2>/dev/null so only stdout (benchmark JSON) is
captured; stderr diagnostics are discarded.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.06%. Comparing base (bef0b90) to head (61e62c1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3925   +/-   ##
=======================================
  Coverage   90.06%   90.06%           
=======================================
  Files         226      226           
  Lines        7223     7223           
=======================================
  Hits         6505     6505           
  Misses        718      718           

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