Apply the log level based on the value of the RUST_LOG environment variable

[kimhanbeom]

• Currently, when RUST_LOG is used as the giganto execution option, there is an issue where the value of that option is not applied to the log level.
• The reason for the issue is that the log level read from the environment variable is overwritten by the INFO level.

      let layer_file = fmt::Layer::default()
          .with_ansi(false)
          .with_target(false)
          .with_writer(file_writer)
          .with_filter(EnvFilter::from_default_env().add_directive(LevelFilter::INFO.into())); // <-- 여기서 INFO로 덮어쓰고 있습니다.

• The following modification is required to allow the default log level to be INFO but overwritten by the RUST_LOG level read from the environment variable.

      let layer_file = fmt::Layer::default()
          .with_ansi(false)
          .with_target(false)
          .with_writer(file_writer)
          .with_filter(
              EnvFilter::builder()
                  .with_default_directive(LevelFilter::INFO.into())
                  .from_env_lossy(),
          );