services.postgres: move the generated statement at the top of the file

    services.postgresql.dataDir = mkDefault "/var/lib/postgresql/${cfg.package.psqlSchema}";

    services.postgresql.authentication = mkAfter

    services.postgresql.authentication = mkMerge [

      (mkBefore "# Generated file; do not edit!")

      (mkAfter

      ''

        # Generated file; do not edit!

        # default value of services.postgresql.authentication

        local all all              peer

        host  all all 127.0.0.1/32 md5

        host  all all ::1/128      md5

      '';

      '')

    ];

    users.users.postgres =

      { name = "postgres";