run_mojo can't find ArtifactStore.packageRoot

The `run_mojo` command doesn't integrate with `FlutterCommand` and doesn't
understand how to download its toolchain components ahead of time. Eventually
we should teach `run_mojo` how to integrate with the `Toolchain` class, but
until then, we can fix the regression by eagerly setting
`ArtifactStore.packageRoot` again.

Fixes https://github.com/domokit/mojo/issues/475