Build script does not honor RUSTC_WRAPPER

[RalfJung]

This is the same issue as djc/rustc-version-rs#45 and rust-lang/libc#3772: the build script invokes rustc without properly applying the RUSTC_WRAPPER environment variable. That's a bug, it means tools that set RUSTC_WRAPPER to redirect rustc invocations somewhere else will not work properly. One such tool is rustc bootstrap; this issue blocks rust-lang/rust#127682.

(It also seems like this crate automatically enables nightly features when built on a nightly compiler. That's a bad idea, see rust-lang/rust#120804, rust-lang/rust#124339, SergioBenitez/version_check#23.)