rustbuild: Pass -Zsave-analysis during tests

This is needed to ensure that the crates during a normal build are
shared with the crates during testing, otherwise they'll end up hasing
differently and we'll recompile crates like `core` during tests.

src/bootstrap/builder.rs

@@ -1134,8 +1134,7 @@ pub fn cargo(
             }
         }
 
-        if (cmd == "build" || cmd == "rustc")
-            && mode == Mode::Std
+        if mode == Mode::Std
             && self.config.extended
             && compiler.is_final_stage(self)
         {