Auto merge of #1406 - RalfJung:miri-as-rustc, r=RalfJung

cargo-miri: never invoke rustc

Always go through 'MIRI_BE_RUSTC=1 miri' instead. This is based on @oli-obk's great idea to add a way to make Miri behave like rustc, which already helped us in https://github.com/rust-lang/miri/pull/1405. Now it means in `cargo-miri` we run *all* crates through the same binary, and use the env var to determine if we compile or interpret them. This makes sure the compiler is consistent.

The `rustc` binary of the current toolchain is now not used at all, only the `miri` binary is. In particular this means we can kill the sysroot consistency check. :)