rustbuild: Only -Zsave-analysis for libstd

Don't pass the flag when we're compiling the compiler or other related tools

src/bootstrap/lib.rs

@@ -482,7 +482,9 @@ impl Build {
             }
         }
 
-        if self.config.extended && compiler.is_final_stage(self) {
+        if mode == Mode::Libstd &&
+           self.config.extended &&
+           compiler.is_final_stage(self) {
             cargo.env("RUSTC_SAVE_ANALYSIS", "api".to_string());
         }