code: func (r *printRun) RegisterIDFlag() {
  	r.Flags.BoolVar(&r.id, "id", false, doc(`
  		Print only build ids.
  
  		Intended for piping the output into another bb subcommand:
  			bb ls -cl myCL -id | bb cancel
  	`))
  }
docs: // RegisterIDFlag registers -id flag.