include environment in default outputPath

We are testing concurrent deployments using threads. We are doing this locally, but I figured it might make sense as the default:

```js
outputPath: `tmp/deploy-dist-${environment}`
```

I started making a PR, but since the environment is dynamic and the defaults are defined as static, I couldn't figure out how to get it to work, especially with the EMBER_CLI_DEPLOY_REUSE_BUILD section. Also, how would you detect if someone already customised the outputPath before appending the environment?

首先追踪 outputPath 的默认值是如何定义的，以及 EMBER_CLI_DEPLOY_REUSE_BUILD 部分如何使用它。确定基于环境的动态默认值应如何与自定义的 outputPath 值和并发 deployments 交互；完成的标准是，该行为已针对默认路径和自定义路径进行了规范说明并覆盖。

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