ember try and monorepos doesn't always do what we expect

[Dhaulagiri]

A side effect of observing this run of ember-flight-icons only failing for the ember try scenarios has revealed what I think is a subtle problem with how ember-try is working in our monorepo setup.

What is happening

ember-try is running yarn from within the given package which is not how the yarn workspace "works". The install should always happen at the root.

What should happen

ember-try runs yarn from the workspace root and then runs the actual scenarios from within the specific page.

[Dhaulagiri]

We could explore ember-try's workspace feature but I'm not sure if that is the best approach or not

[Dhaulagiri]

ember-cli/ember-try#597 helps explain why this is happening (in part)