Weird how no one has mentioned that if the Pennsylvania legislature were to try to amend Pennsylvania's statute on elector selection Wolf could just veto the bill and that's the end.
Can he? I don't think the Constitution even mentions state level vetos. It does say the manner of the appointment of the electors is determined by the legislature. Unless it's just assumed that's part of legislative action.
I think the idea is the following: PA almost certainly has a law that its electors will be appointed in accordance with the popular vote in the state, and that represents the "determination of the state legislature" that the Constitution specifies. Under that interpretation, in order for the legislature to change its determination, it would have to pass a new law overriding this law, which of course Wolf (D-PA) would veto.