fix for prefixed ● in systemd output

[Skywalker-11]

With debian bullseye (systemd 247.3-6) the systemd output has a ● prefix for failed units because of which the parsing of the output fails:

# systemctl --failed --no-legend 
● nagios-nrpe-server.service loaded failed failed Nagios Remote Plugin Executor

[helmo]

I found a simpler solution for this just add --plain to the systemctl command.