message: nydus-image: change default compressor to zstd
  The zstd compressor may achieve better compression ratio, so it's
  suitable as default configuration.
diff: @@ -246,7 +246,7 @@ fn prepare_cmd_args(bti_string: &'static str) -> App {
  .short('c')
  .help("algorithm to compress image data blob:")
  .required(false)
  - .default_value("lz4_block")
  + .default_value("zstd")
  .value_parser(["none", "lz4_block", "gzip", "zstd"]),
  )
  .arg(
  