copy-worker: Copy, as opposed to moving files

The background to this is that gnome-initial-setup sets various GSettings in the gnome-initial-setup user's profile, and then these are adopted by the new user's account by the copy-worker which runs as the very first thing in the user session. This commit is here because store demo mode deletes every file in the demo user's home directory when they log out, so that the next user in the store gets a pristine demo session. It follows that the copy-worker will need to adopt the settings from the gnome-initial-setup user more than once.

This is useful on Endless OS and other OSs which add ‘demo’-style modes. It’s harmless elsewhere.