fix: Use proper thread pool in cumulative_eval

[ritchie46]

fixes #18884

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 13 lines in your changes missing coverage. Please review.

Project coverage is 79.86%. Comparing base (56bdc4e) to head (778ad60).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-lazy/src/dsl/eval.rs	0.00%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18885      +/-   ##
==========================================
+ Coverage   79.85%   79.86%   +0.01%     
==========================================
  Files        1520     1520              
  Lines      206543   206545       +2     
  Branches     2906     2906              
==========================================
+ Hits       164926   164956      +30     
+ Misses      41069    41041      -28     
  Partials      548      548              

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