Updated to use the override sha flag in the repo_util

Bug: https://github.com/flutter/flutter/issues/119012
Change-Id: I1092ef7e54efb190abd21c0edfad07fc1af4e3ae
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/38222
Reviewed-by: Godofredo Contreras <godofredoc@google.com>
Commit-Queue: Ricardo Amador <ricardoamador@google.com>
diff --git a/recipes/cocoon/cocoon.py b/recipes/cocoon/cocoon.py
index b6968e0..2f14fbf 100644
--- a/recipes/cocoon/cocoon.py
+++ b/recipes/cocoon/cocoon.py
@@ -36,6 +36,7 @@
       flutter_path,
       ref=flutter_git_ref,
       url='https://github.com/flutter/flutter',
+      override_sha=True
   )
 
   # Read yaml file