add comment about breakage

Author: jyn514

src/bootstrap/src/bin/rustc.rs

@@ -50,6 +50,7 @@ fn main() {
         ("RUSTC_REAL", "RUSTC_LIBDIR")
     };
 
+    // FIXME: kobzol broke #111323 by accident in #116448
     let sysroot = env::var_os("RUSTC_SYSROOT").expect("RUSTC_SYSROOT was not set");
     let on_fail = env::var_os("RUSTC_ON_FAIL").map(Command::new);