Use restrict key option and relative borg command

No functional change; the "restrict" key option is a shorthand for:

- no-agent-forwarding
- no-port-forwarding
- no-X11-forwarding
- no-pty
- no-user-rc

It was added in OpenSSH 7.2 (2016-02-29) as a convenient way to specify
an authorized key should have "all current and future key restrictions"
applied to it.

Also switch to a relative borg command since its location is not really
standardized; on rsync.net it appears to be located under usr/local/bin
(though /usr/bin/borg works too, even if it doesn't exist!) and Hetzner
just forces its own command, ignoring ours. 

The Borg documentation seems to agree with both the above alterations:

[1] https://borgbackup.readthedocs.io/en/stable/usage/serve.html