openntpd: make -s flag work

after seeing
`adjtime failed: Invalid argument` in my syslog, I tried using
`ntpd -s` but it would trigger
`/etc/ntpd.conf: No such file or directory`
see https://github.com/NixOS/nixpkgs/issues/31885

Instead of running the daemon with a specific config file, use the
standard file so that user are able to use the ntp executable without
having to look for the current config file.