msvc: fix detect_showincludes_prefix with MSBuild

[AJIOB]

The problem usually occurs when VS_UNICODE_OUTPUT is set. This is a @AudranDoublet #1835 and @exoosh #2483 fixes with working tests: thank you!

Should fix issues #1830 and #909

[codecov-commenter]

Codecov Report

❌ Patch coverage is 76.92308% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.17%. Comparing base (951e247) to head (4f1f083).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/compiler/msvc.rs	72.72%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2544      +/-   ##
==========================================
- Coverage   71.17%   71.17%   -0.01%     
==========================================
  Files          64       64              
  Lines       35592    35588       -4     
==========================================
- Hits        25334    25328       -6     
- Misses      10258    10260       +2     

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[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing AJIOB:VS_UNICODE_OUTPUT_fix_2 (4f1f083) with main (f66b044)}

Summary

✅ 27 untouched benchmarks
⏩ 2 skipped benchmarks¹

Footnotes

1. 2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[sylvestre]

could you please add a test to make sure we don't regress? thanks

[AJIOB]

could you please add a test to make sure we don't regress? thanks

@sylvestre , done

[sylvestre]

could you please add a test to make sure we don't regress? thanks

@sylvestre , done

to make sure i understand, what you added in the CI was previously failing to build, right ?
thanks

[AJIOB]

Yes, it previously fails.