[ci] give 3 chances to `brew update`

extra/azure-pipelines/build-mac.yml

@@ -14,7 +14,7 @@ jobs:
         submodules: recursive
       - script: |
           set -ex
-          brew update
+          brew update || brew update || brew update
           brew bundle --file=tests/Brewfile --no-upgrade
         displayName: Install dependencies
       - template: install-neko-snapshot.yaml