fix: webpack require failed in config file

[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 #6225 (7a979fb) into master (00074b9) will decrease coverage by 0.32%.
The diff coverage is 17.64%.

@@            Coverage Diff             @@
##           master    #6225      +/-   ##
==========================================
- Coverage   84.73%   84.40%   -0.33%     
==========================================
  Files         152      153       +1     
  Lines        3453     3470      +17     
  Branches      938      942       +4     
==========================================
+ Hits         2926     2929       +3     
- Misses        517      531      +14     
  Partials       10       10              

Impacted Files	Coverage Δ
packages/umi/src/initWebpack.ts	0.00% <0.00%> (ø)
packages/bundler-webpack/src/requireHook.ts	78.94% <100.00%> (+3.94%)	⬆️

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