Update tests for code_utils and use ThreadPoolExecutor for code execution in windows

[ekzhu]

Why are these changes needed?

AutoGen loves windows.

1. Address This MICROSOFT AUTOGEN was built and made for Mac ONLY ... Nothing works on Windows. SIGALRM is not even compatible with WINDOWS. #1277. Use ThreadPoolExecutor on Windows as well.
2. Clean up the test_execute_code test. Remove try-catch blocks around import docker as it is now a core dependency.
3. Use --skip-docker to explicitly skip docker-related tests where use_docker=True, avoiding the docker server problem on GitHub action's window instances.
4. Use env setting in build.yml workflow file as it is the standard.

Related issue number

#1277

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

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (392d381) 34.30% compared to head (f879ada) 46.58%.

Files	Patch %	Lines
autogen/code_utils.py	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1472       +/-   ##
===========================================
+ Coverage   34.30%   46.58%   +12.28%     
===========================================
  Files          42       42               
  Lines        5110     5102        -8     
  Branches     1169     1236       +67     
===========================================
+ Hits         1753     2377      +624     
+ Misses       3213     2524      -689     
- Partials      144      201       +57     

Flag	Coverage Δ
unittests	46.55% <88.88%> (+12.28%)	⬆️

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