Rollup merge of #104046 - RalfJung:run-miri-run, r=oli-obk

bootstrap: add support for running Miri on a file

This enables:
```
./x.py run src/tools/miri --stage 0 --args src/tools/miri/tests/pass/hello.rs
```
That can be super helpful for debugging.

Also avoid sharing the Miri sysroot dir with a system-wide (rustup-managed) installation of Miri.

Fixes rust-lang/rust#76666

Author: compiler-errors