Destructured variables aren't re-evaluated during adaptive marking or explore mode

[christianp]

An example with adaptive marking: https://numbas.mathcentre.ac.uk/question/share/view/5be5a8cd-0fc2-4f6f-a938-1a19ecf938b1
I have a variable n, and then x,y := [2n,3n]. The variable n is replaced in adaptive marking for part b, but x and y aren't re-evaluated.
The same happens in explore mode, when a variable value is replaced during a next part move.