Fix disabling Chess960

[FitzOReilly]

fixes #236

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.50%. Comparing base (a0e6349) to head (962dd66).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #237      +/-   ##
==========================================
+ Coverage   89.45%   89.50%   +0.04%     
==========================================
  Files          80       80              
  Lines       12589    12591       +2     
==========================================
+ Hits        11262    11269       +7     
+ Misses       1327     1322       -5     

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