Auto merge of rust-lang#2806 - saethlin:better-install, r=RalfJung

Install binaries to the miri toolchain's sysroot

The default install produces this behavior:
```
$ cargo +miri miri --version
miri 0.1.0 (0ba1f4a0 2023-03-05)
$ cargo +nightly miri --version
miri 0.1.0 (0ba1f4a0 2023-03-05)
```
Which is not good. We've effectively erased the toolchain selection, and users may reasonably conclude that their rustup install is broken.

After this change, we now get this:
```
$ cargo +miri miri --version
miri 0.1.0 (0ba1f4a0 2023-03-05)
$ cargo +nightly miri --version
miri 0.1.0 (f63ccaf 2023-03-06)
```

Thanks `@jyn514` who all but wrote this for me.

Author: bors