ci: Run brew update on macOS

Author: real-or-random

.cirrus.yml

@@ -135,6 +135,7 @@ task:
   ##   - rm /tmp/.com.apple.dt.CommandLineTools.installondemand.in-progress
   ##
   brew_valgrind_pre_script:
+    - brew update
     - brew config
     - brew tap --shallow LouisBrunner/valgrind
     # Fetch valgrind source but don't build it yet.