Auto merge of #2259 - RalfJung:cargo-rustc, r=RalfJung
Auto merge of #2259 - RalfJung:cargo-rustc, r=RalfJung

avoid setting both RUSTC and RUSTC_WRAPPER

Fixes https://github.com/rust-lang/miri/issues/2238

rust-lang/miri,rust-lang/miri,rust-lang/miri,rust-lang/miri