update dotnet workflow

[LittleLittleCloud]

Why are these changes needed?

Related issue number

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.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (81fc749) 37.80% compared to head (2d2b6cc) 37.80%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1669   +/-   ##
=======================================
  Coverage   37.80%   37.80%           
=======================================
  Files          50       50           
  Lines        5727     5727           
  Branches     1297     1297           
=======================================
  Hits         2165     2165           
  Misses       3390     3390           
  Partials      172      172           

Flag	Coverage Δ
unittests	37.80% <ø> (ø)

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[ekzhu]

LGTM. I went through the tests haven't found any echoing of secrets.