cmake: stub zephyr_set(... SCOPE ...) function in package helper

Fixes: #63011

Custom targets are not available in script mode, which cause snippet
loading to fail when using package helper.

Lookup of snippets is supported by package helper, thus allowing twister
to filter test cases based on snippets.

However, loading of snippets itself is not supported as snippets make
use of Zephyr scoping, which uses custom targets, something that is not
available in script mode.

Therefore overload the `zephyr_set()` function, so that CMake package
helper can be used together with snippets.

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