fix(mini-runner): 修复 app 样式打包错误的问题

before:

```
// app css
@import base
// app css
@import taro
```

after

```
// app css
@import base
@import taro
```

packages/taro-mini-runner/src/plugins/MiniPlugin.ts

@@ -819,11 +819,11 @@ export default class TaroMiniPlugin {
 
  const originSource: string = assets[appStyle].source()
  const source = new ConcatSource()
+ source.add(originSource)
 
  Object.keys(assets).forEach(assetName => {
  const fileName = path.basename(assetName, path.extname(assetName))
  if (REG_STYLE.test(assetName) && this.options.commonChunks.includes(fileName)) {
- source.add(originSource)
  source.add('\n')
  source.add(`@import ${JSON.stringify(urlToRequest(assetName))};`)
  assets[appStyle] = {