fix: support windows free-threading

[henryiii]

Fixing the logic for finding a library to link to. Python fills the LIBRARY slot with .dll for some reason. CMake was fixing this by finding the .lib anyway, but now we feed it the right library to begin with, which fixes free-threading builds. Tests are currently just downstream in scikit-build-sample-projects until there's an easier way to get a free-threaded build in GHA, especially for Windows.

[codecov]

Codecov Report

Attention: Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.

Project coverage is 81.95%. Comparing base (33d61f1) to head (37fe3b5).
Report is 88 commits behind head on main.

Files with missing lines	Patch %	Lines
src/scikit_build_core/builder/sysconfig.py	72.72%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #759      +/-   ##
==========================================
- Coverage   81.98%   81.95%   -0.04%     
==========================================
  Files          68       68              
  Lines        3903     3913      +10     
==========================================
+ Hits         3200     3207       +7     
- Misses        703      706       +3     

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