cmake: use zephyr_file_copy(...) instead of file(COPY_FILE ...)

`file(COPY_FILE ...)` is from CMake 3.21.
Zephyr still supports CMake 3.20, so use zephyr_file_copy(...)
instead of `file(COPY_FILE ...)`.

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