panic: send on closed channel

goroutine 97 [running]:
github.com/bojand/ghz/load.(*StepWorkerTicker).Run.func1()
	/home/runner/work/ghz/ghz/load/worker_ticker.go:86 +0x1d7
created by github.com/bojand/ghz/load.(*StepWorkerTicker).Run in goroutine 81
	/home/runner/work/ghz/ghz/load/worker_ticker.go:79 +0x19c