fix: exportStatic no work on windows

[chenshuai2144]

Checklist

• npm test passes
• tests are included
• documentation is changed or added
• commit message follows commit guidelines

Description of change

• any feature?
• close https://github.com/umijs/umi/ISSUE_URL

[codecov]

Codecov Report

Merging #6469 (52d7c4e) into master (0867356) will increase coverage by 0.13%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##           master    #6469      +/-   ##
==========================================
+ Coverage   84.76%   84.89%   +0.13%     
==========================================
  Files         157      157              
  Lines        3583     3588       +5     
  Branches      986      987       +1     
==========================================
+ Hits         3037     3046       +9     
+ Misses        536      532       -4     
  Partials       10       10              

Impacted Files	Coverage Δ
...eset-built-in/src/plugins/features/exportStatic.ts	28.57% <90.00%> (+10.51%)	⬆️

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[sorrycc]

看起来对非 win 用户会有 break change，产物格式变了？只针对 win 做变更吧。