fix: use dev variables dumi no work

[xiaohuoni]

使用一些环境变量之后，api 的 cwd 是动态的，比如使用 APP_ROOT=dumi dumi dev
正确的 cwd 是 /Users/xiaohuoni/Desktop/cra/dumi-demo-cra/dumi
dumi 中使用 process.cwd() 获取到的是 /Users/xiaohuoni/Desktop/cra/dumi-demo-cra

Close: #121
Close: #161

[codecov]

Codecov Report

Merging #163 into master will increase coverage by 0.06%.
The diff coverage is 37.50%.

@@            Coverage Diff             @@
##           master     #163      +/-   ##
==========================================
+ Coverage   49.52%   49.59%   +0.06%     
==========================================
  Files          73       73              
  Lines        1462     1464       +2     
  Branches      499      502       +3     
==========================================
+ Hits          724      726       +2     
  Misses        734      734              
  Partials        4        4              

Impacted Files	Coverage Δ
packages/preset-dumi/src/plugins/core.ts	0.00% <0.00%> (ø)
packages/preset-dumi/src/routes/getRouteConfig.ts	0.00% <0.00%> (ø)
...es/preset-dumi/src/routes/getRouteConfigFromDir.ts	100.00% <100.00%> (ø)
...ackages/preset-dumi/src/transformer/remark/yaml.ts	70.00% <100.00%> (+1.57%)	⬆️

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