selfoss: don't hard-code nginx user in service

      user = mkOption {

        type = types.str;

        default = "nginx";

        default = config.services.nginx.user;

        defaultText = lib.literalExpression "config.services.nginx.user";

        description = ''

          User account under which both the service and the web-application run.

        '';

  config = mkIf cfg.enable {

    services.phpfpm.pools = mkIf (cfg.pool == "${poolName}") {

      ${poolName} = {

        user = "nginx";

        user = config.services.nginx.user;

        settings = mapAttrs (name: mkDefault) {

          "listen.owner" = "nginx";

          "listen.group" = "nginx";

          "listen.owner" = config.services.nginx.user;

          "listen.group" = config.services.nginx.group;

          "listen.mode" = "0600";

          "pm" = "dynamic";

          "pm.max_children" = 75;