Fix int overflow on x32 processors causing panic (#3810)

During the roll, "duration" is converted to int, causing overflow on x32 processors. The value may be very small or negative, causing a panic in rand.Intn.

app/observatory/burst/healthping.go

@@ -156,7 +156,7 @@ func (h *HealthPing) doCheck(tags []string, duration time.Duration, rounds int)
 		for i := 0; i < rounds; i++ {
 			delay := time.Duration(0)
 			if duration > 0 {
-				delay = time.Duration(dice.Roll(int(duration)))
+				delay = time.Duration(dice.RollInt63n(int64(duration)))
 			}
 			time.AfterFunc(delay, func() {
 				errors.LogDebug(h.ctx, "checking ", handler)