apply #[allow(unused)] on bin_helpers::parse_rustc_stage

Signed-off-by: onur-ozkan <work@onurozkan.dev>

src/bootstrap/src/utils/bin_helpers.rs

@@ -18,6 +18,7 @@ pub(crate) fn parse_rustc_verbose() -> usize {
 /// Parses the value of the "RUSTC_STAGE" environment variable and returns it as a `String`.
 ///
 /// If "RUSTC_STAGE" was not set, the program will be terminated with 101.
+#[allow(unused)]
 pub(crate) fn parse_rustc_stage() -> String {
     std::env::var("RUSTC_STAGE").unwrap_or_else(|_| {
         // Don't panic here; it's reasonable to try and run these shims directly. Give a helpful error instead.