Auto merge of rust-lang#3599 - saethlin:quiet-when-no-change, r=RalfJung

Don't print unnecessary sysroot messages

Currently, when running `cargo miri setup`, we always print that a sysroot is being prepared, even if we just bail out of building after checking the hash. So that message is wrong; we didn't actually prepare a sysroot.

We also always print the preparing message for `cargo miri run`, even if no sysroot is prepared.

With this PR, `cargo miri run` prints no sysroot messages when an existing one is reused, and when a redundant `cargo miri setup` is requested, we print:
```
A sysroot for Miri is already available in `/home/ben/.cache/miri`.
```

Author: bors