Use 'systemctl reboot' instead of 'reboot' with systemd

'systemctl reboot' will always work when systemd is the init system. 'reboot' no longer works when systemd is the init system but sysvinit-core has been installed, as the latter expects some legacy interfaces (/run/initctl) to exist, which they don't anymore with modern systemd, as they are deprecated and removed with v258.

root@host:~# reboot WARNING: could not determine runlevel - doing soft reboot (it's better to use shutdown instead of reboot from the command line) shutdown: /run/initctl: No such file or directory init: /run/initctl: No such file or directory

https://ci.debian.net/packages/r/runit/testing/amd64/63554762/