Quoting from the Rune Knight's Skye (Cloud Rune)
In addition, when you or a creature you can see within 30 feet of you is hit by an attack roll, you can use your reaction to invoke the rune and cause that attack to target a different creature within 30 feet of you (other than the attacker), using the same roll. This magic can transfer the attack regardless of the attack’s range. Once you invoke the rune, you can’t do so again until you finish a short or long rest.
Does that mean the Rune Knight chooses the new target for the attack?