bootstrap: Make clean respect dry-run

src/bootstrap/clean.rs

@@ -85,6 +85,10 @@ fn run(self, builder: &Builder<'_>) -> Self::Output {
 }
 
 fn clean_default(build: &Build, all: bool) {
+    if build.config.dry_run() {
+        return;
+    }
+
     rm_rf("tmp".as_ref());
 
     if all {