fix default nproc for macos and windows

[njzjz]

No description provided.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (1954d5d) 89.91% compared to head (260ec11) 89.51%.

Files	Patch %	Lines
mddatasetbuilder/datasetbuilder.py	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #193      +/-   ##
==========================================
- Coverage   89.91%   89.51%   -0.41%     
==========================================
  Files           7        7              
  Lines         615      620       +5     
==========================================
+ Hits          553      555       +2     
- Misses         62       65       +3     

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