calculate step duration in a panic-safe way

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

src/bootstrap/src/core/builder/mod.rs

@@ -1534,7 +1534,7 @@ impl<'a> Builder<'a> {
             let out = step.clone().run(self);
             let dur = start.elapsed();
             let deps = self.time_spent_on_dependencies.replace(parent + dur);
-            (out, dur - deps)
+            (out, dur.saturating_sub(deps))
         };
 
         if self.config.print_step_timings && !self.config.dry_run() {