fix: support multiplexed path

[henryiii]

Pulling out this fix from #880 for inclusion in a patch release.

[codecov]

Codecov Report

Attention: Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 83.86%. Comparing base (a3df152) to head (a5fdfa7).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/scikit_build_core/builder/builder.py	62.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #896      +/-   ##
==========================================
- Coverage   83.90%   83.86%   -0.05%     
==========================================
  Files          74       74              
  Lines        4387     4393       +6     
==========================================
+ Hits         3681     3684       +3     
- Misses        706      709       +3     

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