on rustc step, return immediately if --keep-stage 0 was given

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

Author: onur-ozkan

src/bootstrap/src/core/build_steps/compile.rs

@@ -843,6 +843,11 @@ impl Step for Rustc {
         let compiler = self.compiler;
         let target = self.target;
 
+        // Return immediately if `--keep-stage 0` was given.
+        if compiler.stage == 0 && builder.config.keep_stage.contains(&compiler.stage) {
+            return;
+        }
+
         // NOTE: the ABI of the beta compiler is different from the ABI of the downloaded compiler,
         // so its artifacts can't be reused.
         if builder.download_rustc() && compiler.stage != 0 {