Clippy does its own runtime detection of the sysroot, which was
incorrect in this case (it used the beta sysroot). This overrides the
sysroot to use `stage0-sysroot` instead.

- Get `x.py clippy` to work on nightly
- Give a nice error message if nightly clippy isn't installed

Not 100% sure this will _always_ work, but it works currently.

Here's the error for rustdoc:

```
Checking rustdoc artifacts (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
error: no library targets found in package `rustdoc-tool`
```