apply RUSTC_WRAPPER when querying rustc version

[RalfJung]

Fixes #735

The alternative would have been to depend on https://crates.io/crates/rustc_version, but that would have been a larger change.

[RalfJung]

@stepancheg did you have any chance to look at this? :)
This crate shows up in the rustc compiler workspace, which makes this a blocking issue for rust-lang/rust#127682.

[stepancheg]

I don't fully understand it, but someone else asked me to merge it, so this is it.

[stepancheg]

Actually, I removed nightly detection from build.rs of protobuf crate, because it is not needed for the crate itself, only for benches, and I moved benches outside.

[RalfJung]

That's even better, thanks :-)