secretFiles are not chowned correctly in services using DynamicUser

[Mic92]

When User/Group on a systemd service using DynamicUser are set it will break:

serviceConfig = {             
  Restart = "always";         
  DynamicUser = true;         
  User = "grafana-agent";     
  Group = "grafana-agent"; 
...
}

Jun 01 16:26:51 vault-00 5vhm1wk4czcva1xj4v79rszsaparjy37-precreate-dirs-for-grafana-agent[341435]: + chown grafana-agent:grafana-agent /tmp/detsys-vault/
Jun 01 16:26:51 vault-00 5vhm1wk4czcva1xj4v79rszsaparjy37-precreate-dirs-for-grafana-agent[341438]: chown: invalid user: ‘grafana-agent:grafana-agent’

But also if the User/Group field is not set it will not chown the secret at all.