cmake: change check_init_priorities target into a post build command

Fixes: #70359

Remove check_init_priorities target and instead create a post build
command and place the post build command on native_runner_executable, if
that target is the final target, else use the normal post build list for
Zephyr build target.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>