Set RUSTC_WRAPPER instead of RUSTC when invoking Cargo
Set RUSTC_WRAPPER instead of RUSTC when invoking Cargo

Some build scripts rely on the RUSTC binary being the actual compiler
(e.g. parsing the output of 'RUSTC --version'). To prevent clippy from
breaking these build scripts, this commit sets RUSTC_WRAPPER instead.
This will cause Cargo to leave RUSTC unchanged, making the use of clippy
transparent to build scripts.

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust