Merge #1074: ci: Retry brew update a few times to avoid random failures

e51ad3b ci: Retry `brew update` a few times to avoid random failures (Tim Ruffing)
b1cb969 ci: Revert "Attempt to make macOS builds more reliable" (Tim Ruffing)

Pull request description:

ACKs for top commit:
  jonasnick:
    ACK e51ad3b

Tree-SHA512: cb0b81ac8d81fe8ea58afa7382d3f922bd4eb713645c5d0b99f9de963c9906273f5d573a9272e8f6cdb16ffcca5e162c088cc2b0772278f68930f8cb726824be

Author: jonasnick

.cirrus.yml

@@ -135,9 +135,8 @@ task:
   ##   - rm /tmp/.com.apple.dt.CommandLineTools.installondemand.in-progress
   ##
   brew_valgrind_pre_script:
-    # Reinstall brew. We could do `brew update` instead but that often fails.
-    - /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/uninstall.sh)"
-    - /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"
+    # 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.