Now that we apply a strong blur effect to the background, it doesn't
make too much sense to use a separate lock-screen background: It will
be mostly unrecognizable anyway.
The alternative would be to turn off the blur effect if a different
background is used (or have a hidden setting for that), but that would
then imply that we must keep the contents readable without blur.
Let's avoid that rabbit hole and just re-use the regular background.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/1001
0afac36713