Add workaround for testing until Xcode 13.1 VM is enabled

https://github.com/wordpress-mobile/WordPress-iOS/blob/e53e3c3c696079eb9e1cd0384b934907034a8771/.buildkite/commands/build-for-testing.sh#L3-L5

Author: derekblank

.buildkite/commands/build-for-testing.sh

@@ -1,5 +1,9 @@
 #!/bin/bash -eu
 
+# FIXIT-13.1: Temporary fix until we're on the Xcode 13.1 VM
+echo "--- :rubygems: Fixing Ruby Setup"
+gem install bundler
+
 echo "--- :rubygems: Setting up Gems"
 install_gems