fix(miniflux): use systemctl to check service instead of file path

The miniflux.service file is installed by the deb package at
/usr/lib/systemd/system/ not /etc/systemd/system/, causing the
update script to fail with 'No Miniflux Installation Found!'.

Using 'systemctl is-enabled' is more robust and works regardless
of where the service file is located.

ct/miniflux.sh

@@ -23,7 +23,7 @@ function update_script() {
   header_info
   check_container_storage
   check_container_resources
-  if [[ ! -f /etc/systemd/system/miniflux.service ]]; then
+  if ! systemctl -q is-enabled miniflux 2>/dev/null; then
     msg_error "No ${APP} Installation Found!"
     exit
   fi