fix: detect manual generator setting for get_requires

[henryiii]

Fix #778.

[codecov]

Codecov Report

Attention: Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 83.66%. Comparing base (6afb1d6) to head (e86a3dc).
Report is 62 commits behind head on main.

Files with missing lines	Patch %	Lines
src/scikit_build_core/builder/get_requires.py	70.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #840      +/-   ##
==========================================
- Coverage   83.68%   83.66%   -0.02%     
==========================================
  Files          74       74              
  Lines        4204     4212       +8     
==========================================
+ Hits         3518     3524       +6     
- Misses        686      688       +2     

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