Do not inhibit idle in borgmatic.service (#589).

Reviewed-on: https://projects.torsion.org/borgmatic-collective/borgmatic/pulls/589

sample/systemd/borgmatic.service

@@ -61,4 +61,4 @@ LogRateLimitIntervalSec=0
 # Delay start to prevent backups running during boot. Note that systemd-inhibit requires dbus and
 # dbus-user-session to be installed.
 ExecStartPre=sleep 1m
-ExecStart=systemd-inhibit --who="borgmatic" --why="Prevent interrupting scheduled backup" /root/.local/bin/borgmatic --verbosity -1 --syslog-verbosity 1
+ExecStart=systemd-inhibit --who="borgmatic" --what="sleep:shutdown" --why="Prevent interrupting scheduled backup" /root/.local/bin/borgmatic --verbosity -1 --syslog-verbosity 1