Specify `RUST_BACKTRACE` is an env var on `panic`

[estebank]

The current panic message doesn't specify that RUST_BACKTRACE is an env variable, which can be difficult to understand for newcomers.

note: Run with `RUST_BACKTRACE=1` for a backtrace.

[kinnison]

On the assumption that changing this message would not be considered a breaking change, I think this would be fairly easy and I'd like to give it a go to get my feet wet in the compiler codebase.

I would propose changing the message to:

note: Run with `RUST_BACKTRACE=1` in the environment to display a backtrace.

Would that be suitable?