More macOS CI fixes (#2352)

* Revert "Fix macOS CI (#2345)"

This reverts commit aab966d935e2033254abd64a1fae97145dcc3a6a.

* ignore update output
old-commit-hash: e23bb6113b

tools/mac_setup.sh

@@ -1,7 +1,13 @@
 #!/bin/bash -e
 
-echo "Updating Homebrew"
-brew update
+# Install brew if required.
+if [[ $(command -v brew) == "" ]]; then
+    echo "Installing Hombrew"
+    /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install.sh)"
+else
+    echo "Updating Homebrew"
+    brew update || true
+fi
 
 brew install capnp \
              czmq \