Auto merge of rust-lang#3402 - RalfJung:miri-script, r=RalfJung

miri script: build with stable toolchain

`./miri toolchain` sets up a `rustup override miri`. But then if something goes wrong and the `miri` toolchain doesn't work, one can't even run `./miri toolchain` again as building miri-script needs a toolchain...

So let's always use stable to build miri-script, making it override-independent. I assume everyone will have that installed.

Author: bors