fix(module:tree-select): enable overflow-x for virtual scroll

[WwwHhhYran]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Application (the showcase website) / infrastructure changes
• Other... Please describe:

What is the current behavior?

Issue Number: close #9044

What is the new behavior?

support x-scroll when item title is too long

Does this PR introduce a breaking change?

• Yes
• No

Other information

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.95%. Comparing base (2d9ff5f) to head (1b97fee).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9045      +/-   ##
==========================================
+ Coverage   91.94%   91.95%   +0.01%     
==========================================
  Files         559      559              
  Lines       19769    19769              
  Branches     3050     3050              
==========================================
+ Hits        18176    18178       +2     
+ Misses       1266     1265       -1     
+ Partials      327      326       -1     

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