fix: [autogenbench] writing to stdout encoding error in win-os

[lalo]

Missed this part of the change in this other PR #1957

Why are these changes needed?

Related issue number

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• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 37.57%. Comparing base (24418bd) to head (96916db).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2002   +/-   ##
=======================================
  Coverage   37.57%   37.57%           
=======================================
  Files          65       65           
  Lines        6912     6912           
  Branches     1521     1521           
=======================================
  Hits         2597     2597           
  Misses       4090     4090           
  Partials      225      225           

Flag	Coverage Δ
unittests	37.57% <ø> (ø)

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[afourney]

Thanks for fixing this, and the prior PR! LGTM