_parseOptionsEnv() {
    this.options.forEach((option) => {
      if (option.envVar && option.envVar in process.env) {
        const optionKey = option.attributeName();
        // Priority check. Do not overwrite cli or options from unknown source (client-code).
        if (
          this.getOptionValue(optionKey) === undefined ||
          ['default', 'config', 'env'].includes(
            this.getOptionValueSource(optionKey),
          )
        ) {
          if (option.required || option.optional) {
            // option can take a value
            // keep very simple, optional always takes value
            this.emit(`optionEnv:${option.name()}`, process.env[option.envVar]);
          } else {
            // boolean
            // keep very simple, only care that envVar defined and not the value
            this.emit(`optionEnv:${option.name()}`);
          }
        }
      }
    });
  }
Apply any option related environment variables, if option does
not have a value from cli or client code.

@private