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 🤔