zsh: suppress aliases and shell functions when calling systemctl

This prevents any errors in case the user had aliased some arguments
following systemctl.

shell-completion/zsh/_systemctl.in

@@ -153,7 +153,7 @@
 # @todo _systemd-run has a helper with the same name, so we must redefine
 __systemctl()
 {
-    systemctl $_sys_service_mgr --full --legend=no --no-pager --plain "$@" 2>/dev/null
+    command systemctl $_sys_service_mgr --full --legend=no --no-pager --plain "$@" 2>/dev/null
 }