ci: Retry brew update a few times to avoid random failures

Author: real-or-random

.cirrus.yml

@@ -135,7 +135,8 @@ task:
   ##   - rm /tmp/.com.apple.dt.CommandLineTools.installondemand.in-progress
   ##
   brew_valgrind_pre_script:
-    - brew update
+    # Retry a few times because this tends to fail randomly.
+    - for i in {1..5}; do brew update && break || sleep 15; done
     - brew config
     - brew tap LouisBrunner/valgrind
     # Fetch valgrind source but don't build it yet.