Merge #48: ci: update brew before installing packages

16af60a ci: update brew before installing packages (fanquake)

Pull request description:

  Given an older base image is being used, this can avoid instances where package installs fail, due to brew trying to use no-longer existent download locations.

  Will see how long the update step takes, although a few minutes extra time spent in the macOS CI is probably less of an issue here. If it seems too long, I think moving to the Big Sur image would also be fine.

ACKs for top commit:
  sipa:
    utACK 16af60a

Tree-SHA512: 6f95dc5562e2524f3d49247dc00e9dfef073789cad30a43666cf4ffcbf4479727bc9840a26c0cf11645690e6c3f144d5938213de68418bf9f3e94465e8cce214

Author: sipa

.cirrus.yml

@@ -114,6 +114,7 @@ task:
     - env:
         CC: clang
   brew_script:
+    - brew update
     - brew install automake libtool gcc@9
   << : *MERGE_BASE
   test_script: