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
7 jobs for !7679 with victoria-1.35.3-mmap in 1 minute and 28 seconds (queued for 3 seconds)
merge request