fix(PickerGroup): rendering correctly when using v-for

[inottn]

fix #12728

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 89.75%. Comparing base (f578ac2) to head (714d03d).
Report is 2 commits behind head on main.

Files	Patch %	Lines
packages/vant/src/picker-group/PickerGroup.tsx	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12732      +/-   ##
==========================================
- Coverage   89.75%   89.75%   -0.01%     
==========================================
  Files         257      257              
  Lines        6835     6842       +7     
  Branches     1660     1662       +2     
==========================================
+ Hits         6135     6141       +6     
- Misses        373      374       +1     
  Partials      327      327              

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[chenjiahan]

LGTM 👍🏻