nestedvm.cmake: fix accidental use of dynamic scope.

In this platform's set_platform_puzzle_target_properties, I referred
to ${EXENAME} twice, which is not one of the function parameters. It
worked anyway, because CMake has dynamic scope, and that variable was
defined - to the right value - within the local-variable scope of the
calling function. But that wasn't at all what I meant to do!

Renamed it to ${TARGET}, which is the actual parameter name we get
passed.