Update service to be oneshot

 - Infinity start time is default
 - Helps with serial execution
 - Also see openSUSE/snapper#392

In order for this to run properly after `snapper-timeline.service`, it
too must be a `Type=oneshot` service to serially execute this service
afterwards. Otherwise we run into a race condition where this service is
not able to read the newly created lvm snapshots yet.

Author: xenithorb

snapper2borg.service

@@ -3,7 +3,7 @@ Description=Borg backup post snapper snapshots
 After=snapper-timeline.service
 
 [Service]
-Type=simple
+Type=oneshot
 ExecStart=/usr/local/bin/snapper2borg.sh
 
 [Install]