Improve first time Rust experience

[sonnyp]

When running a Rust demo for the first time, a lot of dependencies will get downloaded and compiled. It takes a while.

The second run is very fast because dependencies are shared between projects so this is only a problem on the first Rust "Run" but it might discourage users.

We should consider using sccache or ship prebuilt in Workbench or tell the user about what's going on.