ci: Update brew on macOS

The preinstalled brew is very old and tries to download prebuilt bottles
from a server which is no longer available. Because that will fail, brew
falls back to building our dependencies (e.g., autotools) from source,
which takes very long.

This commit makes sure that brew is updated before we start the build.

We also need to remove the `--shallow` argument from `brew tap`. It
doesn't exist in recent brew versions.

.cirrus.yml

@@ -133,8 +133,9 @@ task:
   ##   - rm /tmp/.com.apple.dt.CommandLineTools.installondemand.in-progress
   ##
   brew_valgrind_pre_script:
+    - brew update
     - brew config
-    - brew tap --shallow LouisBrunner/valgrind
+    - brew tap LouisBrunner/valgrind
     # Fetch valgrind source but don't build it yet.
     - brew fetch --HEAD LouisBrunner/valgrind/valgrind
   brew_valgrind_cache: