Retry mac_unopt one time in presubmit (#56319)

While we mitigate https://github.com/flutter/flutter/issues/157636.

.ci.yaml

diff --git a/.ci.yaml b/.ci.yaml
index 448d669..ddced8b 100644
--- a/.ci.yaml
+++ b/.ci.yaml
@@ -516,6 +516,8 @@
     properties:
       config_name: mac_unopt
       add_recipes_cq: "true"
+      # Retry for flakes caused by https://github.com/flutter/flutter/issues/157636
+      presubmit_max_attempts: "2"
     timeout: 120
 
   - name: Mac mac_ios_engine