nixos: tarsnap - make systemd timer persistent

A machine may not always be active (or online!) when a backup timer
triggers, meaning backups can be missed - now we properly set the
tarsnap timer's Persistent option so systemd will run the command even
when the machine wasn't online at that exact time.

However, we also need to make sure that we can contact the tarsnap
server reliably before we start the backup. So, we attempt to ping the
access endpoint in a loop with a sleep, before continuing.

This fixes #8823.

Signed-off-by: Austin Seipp <aseipp@pobox.com>