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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 https://github.com/rust-lang/rust/issues/76666
