Add support for RUSTC environment variable in libudev-sys/build.rs

[kaazoo]

The command "rustc" is hardcoded here: https://github.com/dcuddeback/libudev-sys/blob/master/build.rs#L28

Please add support for the RUSTC environment variable (see https://doc.rust-lang.org/cargo/reference/environment-variables.html) which is used by cargo to point to an alternative rustc path.

Building packages with Rust 1.80 on Ubuntu 24.04 requires patching build.rs currently, as rustc lives under /usr/bin/rustc-1.80 .