Use utf-8 encoding at subprocess.run()

[giorgossideris]

Use utf-8 encoding for local code execution

Closes #3453

Checks

• 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.

[Hk669]

@giorgossideris thanks for the fix.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 37.46%. Comparing base (6279247) to head (a1a5dd5).
Report is 128 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3454      +/-   ##
==========================================
+ Coverage   32.90%   37.46%   +4.55%     
==========================================
  Files          94       96       +2     
  Lines       10235    10979     +744     
  Branches     2193     2534     +341     
==========================================
+ Hits         3368     4113     +745     
+ Misses       6580     6493      -87     
- Partials      287      373      +86     

Flag	Coverage Δ
unittest	11.94% <ø> (?)
unittests	37.41% <ø> (+4.51%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.