well actually it's a line out - there was a clarificatin (that we have discussed a few times before)
http://laws.worldrugby.org/?domain=10&clarlaw=21&clarification=27
[LAWS]Clarification 2 2006
Following a penalty kick and after the ball was kicked, the ball hits the goal post and goes into touch without having been touched by another player.
What decision should the referee give?
Ruling in Law by the Designated Members of the Rugby Committee
If the penalty kick is for goal, then it is a lineout defending team to throw in.
Law 21.4(d).
If the penalty kick is for touch, therefore no place kick, then it is a lineout attacking team to throw in.
The lineout in either of these situations may not be closer than 5 metres from the goal line. Law 19.4 Exception.[/LAWS]
Ha !
So the answer is
Place for goal accidentally goes into touch -- Lineout
Place kick for goal intentionally goes into touch -- Scrum
The fact that time expired means I have a lot of sympathy with the idea of a PK, but it's not what the Law would dictate.