nixos/tests/nextcloud: follow variable rename (#466054)

          services.nextcloud.config.objectstore.s3 = {

            enable = true;

            bucket = "nextcloud";

            autocreate = true;

            verify_bucket_exists = true;

            key = accessKey;

            secretFile = "${pkgs.writeText "secretKey" secretKey}";

            hostname = "acme.test";