Unrolled build for #142714

Rollup merge of #142714 - fee1-dead-contrib:push-roxtwrlvtzur, r=Kobzol

add comment to `src/bootstrap/build.rs`

I attempted to remove this build script but it's apparently needed. Add a comment for why.

Author: rust-timer

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}");