5ab3177acd
This produced the following warning: ``` DEPRECATION: Project uses feature that was always broken, and is now deprecated since '1.3.0': str.format: Value other than strings, integers, bools, options, dictionaries and lists thereof.. ``` The resulting value of `SHELL_BACKGROUND_IMAGE` was indeed bogus, not containing a proper file path. Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3123>