testing/victoria-metrics: disable mmap by default
Using mmap in go can stall goroutines. VM includes workaround that depends on cgo, but alpine package is built without cgo. Disabling mmap may yield better performance. See: https://github.com/VictoriaMetrics/VictoriaMetrics/blob/baedb25936669179fa726639e267a52d8cdfedc2/lib/fs/copy_mmap_nocgo.go#L7