fix: temporarily disable spilling when training indices on string columns

[westonpace]

Until we upgrade to the next DF release (46) we cannot rely on spilling when working with string data. Users continue to get errors unrelated to the size of the spill pool or the amount of data they have. This disables spilling entirely on string columns (which is the typical workaround) until we get a stable solution.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.71%. Comparing base (f508cbd) to head (a4aa77c).

Files with missing lines	Patch %	Lines
rust/lance/src/index/scalar.rs	90.90%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3469      +/-   ##
==========================================
+ Coverage   78.67%   78.71%   +0.03%     
==========================================
  Files         251      251              
  Lines       92983    92995      +12     
  Branches    92983    92995      +12     
==========================================
+ Hits        73157    73201      +44     
+ Misses      16852    16816      -36     
- Partials     2974     2978       +4     

Flag	Coverage Δ
unittests	78.71% <92.85%> (+0.03%)	⬆️

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