compile_codegen: add warning in build.rs about cargo gpu parsing

Author: Firestar99

crates/rustc_codegen_spirv/build.rs

@@ -11,6 +11,9 @@ use std::process::{Command, ExitCode};
 use std::{env, fs, mem};
 
 /// Current `rust-toolchain.toml` file
+/// WARNING!!! cargo-gpu is new relying on this being a string literal! It will
+/// scan `build.rs` for any line starting with `channel = "..."` to figure out
+/// which toolchain version to use! This also allows backwards compat.
 /// Unfortunately, directly including the actual workspace `rust-toolchain.toml` doesn't work together with
 /// `cargo publish`. We need to figure out a way to do this properly, but let's hardcode it for now :/
 //const REQUIRED_RUST_TOOLCHAIN: &str = include_str!("../../rust-toolchain.toml");