add comment to src/bootstrap/build.rs

src/bootstrap/build.rs

@@ -1,6 +1,7 @@
 use std::env;
 
 fn main() {
+    // this is needed because `HOST` is only available to build scripts.
     let host = env::var("HOST").unwrap();
     println!("cargo:rerun-if-changed=build.rs");
     println!("cargo:rustc-env=BUILD_TRIPLE={host}");