benchmark: Specify passthrough resolver to avoid resolution failures

[arjan-bal]

In #7970, the benchmarking code was changed to use NewClient instead of Dial to create channels. This changed the default resolver from passthrough to dns. The usage fixed in this PR depended on the custom dialer dialing a fixed address irrespective of the resolver. Since the target was empty, the DNS resolver produces an error and causing RPCs to fail.

RELEASE NOTES: N/A

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.18%. Comparing base (b368379) to head (c1f81d9).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8231      +/-   ##
==========================================
+ Coverage   81.99%   82.18%   +0.18%     
==========================================
  Files         410      410              
  Lines       40389    40389              
==========================================
+ Hits        33116    33192      +76     
+ Misses       5893     5839      -54     
+ Partials     1380     1358      -22     

see 29 files with indirect coverage changes

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