perf: Don't infer CSV schema if schema is set

[orlp]

Fixes #27456.

[orlp]

@nameexhaustion Could you review this? Not sure if this has unknown implications.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.85%. Comparing base (1156024) to head (8997fb7).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #27507      +/-   ##
==========================================
- Coverage   81.14%   79.85%   -1.29%     
==========================================
  Files        1834     1834              
  Lines      254891   254896       +5     
  Branches     3177     3177              
==========================================
- Hits       206823   203545    -3278     
- Misses      47246    50529    +3283     
  Partials      822      822              

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