Install the mailutils package in the unattended-upgrades recipe if the mail attribute is set to true

This ensures that the server will be capable of sending the emails after performing the upgrades.

This was inspired by another cookbook that handled unattended-upgrades:
https://github.com/firstbanco/chef-unattended-upgrades/blob/13b0b86386d87ee4d19ae62451ca7f37b9c87d02/recipes/default.rb#L5-L7