CHORE: Fix PerformanceWarning in pivot combine stage

[ChengjieLi28]

What do these changes do?

Fix this warning in pivot op combine stage.

performancewarning: dataframe is highly fragmented. this is usually the result of calling `frame.insert` many times, which has poor performance. consider joining all columns at once using pd.concat(axis=1) instead. to get a de-fragmented frame, use `newframe = frame.copy()`

Related issue number

Fixes #xxxx

Check code requirements

• tests added / passed (if needed)
• Ensure all linting tests pass

[codecov]

Codecov Report

Merging #747 (f1587de) into main (b320ca3) will decrease coverage by 27.67%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##             main     #747       +/-   ##
===========================================
- Coverage   77.29%   49.62%   -27.67%     
===========================================
  Files        1058     1058               
  Lines       79780    79779        -1     
  Branches    16504    12433     -4071     
===========================================
- Hits        61663    39593    -22070     
- Misses      15724    38595    +22871     
+ Partials     2393     1591      -802     

Flag	Coverage Δ
unittests	49.61% <0.00%> (-27.56%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
python/xorbits/_mars/dataframe/base/pivot.py	23.70% <0.00%> (+0.17%)	⬆️

... and 639 files with indirect coverage changes