Make sure the build_e2e.dart script points to the dart sdk that is included with flutter

https://github.com/flutter/devtools/pull/5558#discussion_r1154959777
@kenzieschmoll mentioned the following:
> One thing I just thought of: we should make sure the build_e2e.dart script points (maybe optionally?) to the dart sdk that is included with flutter. I think as is, there will be a dart version mismatch between the devtools server and the dart sdk that we use to run devtools_app 🤔

打开 build_e2e.dart，追踪它如何选择 Dart 可执行文件；将该选择与 Flutter 中捆绑的 Dart SDK 进行比较。当脚本可以使用 Flutter 中包含的 SDK（可能作为可选项）时，即表示完成，这样 DevTools 服务器和 devtools_app 就不会使用不匹配的 Dart 版本运行。

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