Auto merge of #3411 - RalfJung:sysroot, r=RalfJung

Handle Miri sysroot entirely outside the Miri driver

(Extracted from https://github.com/rust-lang/miri/pull/3409)

This entirely moves the responsibility of setting miri-sysroot to whatever *invokes* the Miri driver.  cargo-miri knows whether it is inside rustdoc or not and can adjust accordingly. I previously avoided doing that because there are a bunch of places that are invoking the driver (cargo-miri, the ui test suite, `./miri run`, `./x.py run miri`) and they all need to be adjusted now. But it is also somewhat less fragile as we usually have more information there -- and we can just decide that `./miri run file.rs --sysroot path` is not supported. The advantage of this is that the driver is reasonably clean and doesn't need magic environment variables like MIRI_SYSROOT, and we don't have to fight rustc_driver to use a different default sysroot. Everything is done in cargo-miri (and the other much simpler driver wrappers) where it can hopefully be debugged much better.

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