hydra-module: Use PermissionsStartOnly in init.

Oops, forgot to add this in f75509099a.

This is necessary because we actually want to run the preStart script as
root (because it chmod/chowns stuff and also needs to create the
database using PostgreSQL's superuser) and the actual creation of the
database as user hydra.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

hydra-module.nix

@@ -197,6 +197,7 @@ in
           ''}
         '';
         serviceConfig.ExecStart = "${cfg.package}/bin/hydra-init";
+        serviceConfig.PermissionsStartOnly = true;
         serviceConfig.User = "hydra";
         serviceConfig.Type = "oneshot";
         serviceConfig.RemainAfterExit = true;