How to set outputDir in the vue cli plugin?

### What is expected?

I am developing a vue cli plugin and I need to set outputDir via an environment variable. But I can't write this in the plugin. Is there any other way to set it up?

```
  api.extendPackage({
    vue: {
      outputDir: process.env.NODE_ENV === 'mp' ? './dist/mp/common' : './dist/web'
    }
  })
```