nixos/nextcloud: add occ in path of nextcloud-setup

This makes possible to use nextcloud-occ for setting extra
configuration options.

Example:

  systemd.services.nextcloud-setup = {
    script = mkAfter ''
      nextcloud-occ config:system:set redis 'host' --value '/var/run/redis/redis.sock' --type string
      nextcloud-occ config:system:set redis 'port' --value 0 --type integer
    '';
  };