fix: style-loader runtime lost

[sorrycc]

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 #6224 (031f419) into master (d17ceca) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6224   +/-   ##
=======================================
  Coverage   84.73%   84.73%           
=======================================
  Files         152      152           
  Lines        3453     3453           
  Branches      938      938           
=======================================
  Hits         2926     2926           
  Misses        517      517           
  Partials       10       10           

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