chore: change example bootstrap

[xiaohuoni]

1、修改一下 example bootstrap 命令，已经写好的代码，不应该被脚本重置
2、所有的 example 下的 node_modules 都应该被忽略

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 #6630 (a243a79) into master (07402af) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6630   +/-   ##
=======================================
  Coverage   84.77%   84.77%           
=======================================
  Files         157      157           
  Lines        3599     3599           
  Branches      991      991           
=======================================
  Hits         3051     3051           
  Misses        538      538           
  Partials       10       10           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 07402af...a243a79. Read the comment docs.