fix: Properly broadcast in sort_by

[coastalwhite]

Fixes #7207.

[codecov]

Codecov Report

Attention: Patch coverage is 74.19355% with 8 lines in your changes missing coverage. Please review.

Project coverage is 78.99%. Comparing base (92fd75d) to head (0abe2df).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-expr/src/expressions/sortby.rs	74.19%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20434      +/-   ##
==========================================
- Coverage   79.00%   78.99%   -0.01%     
==========================================
  Files        1566     1566              
  Lines      221034   221050      +16     
  Branches     2510     2510              
==========================================
+ Hits       174618   174624       +6     
- Misses      45842    45852      +10     
  Partials      574      574              

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