use RUSTC_WRAPPER instead of overriding RUSTC

[nivkner]

some crates, including libc, depend on the RUSTC environment variable actually pointing to the rust compiler for version checks.

according to this stack overflow answer the difference in usage should only be that the cargo-miri binary would get rustc passed before the other parameters

[RalfJung]

I doubt miri will be able to execute much of what libc does, but yeah, using RUSTC_WRAPPER seems reasonable.