native_sim build: Fix for APPLICATION_BINARY_DIR!=CMAKE_BINARY_DIR

In some cases, the APPLICATION_BINARY_DIR does not match
the CMAKE_BINARY_DIR, in those cases the native simulator
build would not find the zephyr elf file.
Fix it by using the correct variable.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>