fix: css sourcemap don't work

[ycjcl868]

ref: #5316 , #5818, #5701
Close #5817

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 #6024 (641a009) into master (3e0aaac) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6024      +/-   ##
==========================================
+ Coverage   82.97%   83.00%   +0.02%     
==========================================
  Files         158      158              
  Lines        3495     3495              
  Branches      942      942              
==========================================
+ Hits         2900     2901       +1     
+ Misses        585      584       -1     
  Partials       10       10              

Impacted Files	Coverage Δ
packages/bundler-webpack/src/getConfig/css.ts	92.85% <ø> (ø)
...ackages/bundler-webpack/src/getConfig/getConfig.ts	86.71% <0.00%> (+0.69%)	⬆️

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