code: func getActiveEnv(appPath, appName string) string {
  	val := os.Getenv(strmangle.EnvAppName(appName) + "_ENV")
  	if val != "" {
  		return val
  	}
  
  	contents, err := afero.ReadFile(AppFS, filepath.Join(appPath, AppConfigFilename))
  	if err != nil {
  		return ""
  	}
  
  	var config AppConfig
  
  	_, err = toml.Decode(string(contents), &config)
  	if err != nil {
  		return ""
  	}
  
  	return config.DefaultEnv
  }
docs: // getActiveEnv attempts to get the config.toml environment
  // to load by checking the following, in the following order:
  // 1. environment variable $APPNAME_ENV (APPNAME is envAppName variable value)
  // 2. config.toml default environment field "env"