Fix VoxelGI saving VoxelGIData as a built-in file, despite being prompted to save it to an external file

[MineBill]

This fixes #50029.

It seems like the resource saver will change the VoxelGIData resource path to the external file, save the resource and the revert it to what it was previously (an empty string), essentially creating two resources, one built-in to the scene (which has the actual data) and the external file. I do not know why the resource saver works this way, but the ScriptCreateDialog does the same thing:

String lpath = ProjectSettings::get_singleton()->localize_path(file_path->get_text());
scr->set_path(lpath);
Error err = ResourceSaver::save(scr, lpath, ResourceSaver::FLAG_CHANGE_PATH);
if (err != OK) {
	alert->set_text(TTR("Error - Could not create script in filesystem."));
	alert->popup_centered();
	return;
}

so I copied that.

[Calinou]

Tested locally, it works.

[YuriSizov]

Thanks! And congrats on your first merged Godot pull-request!

Please consider using feature branches in the future. If you make changes directly to your master, it can be hard to resolve problems. Plus, having feature branches allows you to work on several PRs at the same time, should you consider contributing again :)

[YuriSizov]

Cherry-picked for 4.1.2.