ci: update brew before installing packages

fanquake · fanquake · commit 16af60af2b42 · 2021-09-01T11:04:01.000+08:00
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.
diff --git a/.cirrus.yml b/.cirrus.yml
@@ -114,6 +114,7 @@ task:
     - env:
         CC: clang
   brew_script:
+    - brew update
     - brew install automake libtool gcc@9
   << : *MERGE_BASE
   test_script:
