Auto merge of #123192 - RalfJung:bootstrap-test-miri, r=onur-ozkan

Refactor the way bootstrap invokes `cargo miri`

Instead of basically doing `cargo run --manifest-path=<cargo-miri's manifest> -- miri`, let's invoke the `cargo-miri` binary directly. That means less indirections, and also makes it easier to e.g. run the libcore test suite in Miri. (But there are still other issues with that.)

Also also adjusted Miri's stage numbering so that it is consistent with rustc/rustdoc.

This also makes `./x.py test miri` honor `--no-doc`.

And this fixes rust-lang/rust#123177 by moving where we handle parallel_compiler.

Author: bors