Only use the flag for rustc and std

Author: Kobzol

src/bootstrap/src/core/builder/cargo.rs

@@ -1043,7 +1043,9 @@ impl Builder<'_> {
         // Enable usage of unstable features
         cargo.env("RUSTC_BOOTSTRAP", "1");
 
-        cargo.arg("-Zno-embed-metadata");
+        if matches!(mode, Mode::Rustc | Mode::Std) {
+            cargo.arg("-Zno-embed-metadata");
+        }
 
         if self.config.dump_bootstrap_shims {
             prepare_behaviour_dump_dir(self.build);