Roughly retry `brew update` on GitHub Actions

because it has failed often on Travis.

.github/workflows/macos.yml

@@ -19,7 +19,7 @@ jobs:
           sudo /usr/libexec/ApplicationFirewall/socketfilterfw --getglobalstate
       - name: Install libraries
         run: |
-          brew update
+          timeout 300 brew update || timeout 300 brew update || timeout 300 brew update
           brew install gdbm gmp libffi openssl@1.1 zlib ccache autoconf automake libtool readline
       - name: Checkout # not using actions/checkout because it's unstable.
         run: git clone --depth=50 https://github.com/ruby/ruby .