On an OS X laptop, the variable `$HOST` changes a lot depending
on what wifi network you're connected to. This causes a lot
of `~/.ssh/environment-$HOST` files to be created and
causes multiple ssh-agents to created.
Instead, use `scutil --get ComputerName` to get something
more stable.
By default, ssh-agent stores identities forever. It has an option to
set a maximum lifetime for identites (useful to expire passphrase protected
keys). Allow this option to be set using:
zstyle :omz:plugins:ssh-agent lifetime <time>
