Ignore errors of brew update in setup_ccache

It may fail creating some symlinks which we don't care about. See e.g. https://github.com/actions/runner-images/issues/6817

ci/setup_ccache.sh

@@ -13,7 +13,7 @@ if ! command -v ccache &> /dev/null; then
   if [ -f "/etc/debian_version" ]; then
     sudo apt-get install ${NET_RETRY_COUNT:+ -o Acquire::Retries=$NET_RETRY_COUNT} -y ccache
   elif command -v brew &> /dev/null; then
-    brew update > /dev/null
+    brew update > /dev/null || true # Ignore spurious errors, although the next command may fail.
     brew install ccache
   fi
 fi