cosmic-settings: add upstream patch looking up wallpaper in NixOS-specific path
Usually, I prefer waiting for a release from upstream that includes the
commit but there appears that the next release is a few months away. So
add the patch for the time being.
Once we have a new release, the patch won't apply cleanly and we can
remove it then.
authored by Pratham Patel and committed by Alyssa Ross e6064cfc 91b71e39