nixos-artwork: add file path attributes

This makes things so much easier, and we install to
the path that both gnome-backgrounds and
elementary-wallpapers install to.

pkgs/desktops/pantheon/desktop/elementary-default-settings/default.nix

@@ -62,7 +62,7 @@ stdenv.mkDerivation rec {
 
   mesonFlags = [
     "--sysconfdir=${placeholder "out"}/etc"
-    "-Ddefault-wallpaper=${nixos-artwork.wallpapers.simple-dark-gray}/share/artwork/gnome/nix-wallpaper-simple-dark-gray.png"
+    "-Ddefault-wallpaper=${nixos-artwork.wallpapers.simple-dark-gray.gnomeFilePath}"
     "-Dplank-dockitems=false"
   ];
 

pkgs/desktops/pantheon/desktop/elementary-greeter/default.nix

@@ -89,7 +89,7 @@ stdenv.mkDerivation rec {
     # Needed until https://github.com/elementary/greeter/issues/360 is fixed
     (substituteAll {
       src = ./hardcode-fallback-background.patch;
-      default_wallpaper = "${nixos-artwork.wallpapers.simple-dark-gray}/share/artwork/gnome/nix-wallpaper-simple-dark-gray.png";
+      default_wallpaper = "${nixos-artwork.wallpapers.simple-dark-gray.gnomeFilePath}";
     })
   ];
 
@@ -113,7 +113,7 @@ stdenv.mkDerivation rec {
     # Use NixOS default wallpaper
     substituteInPlace $out/etc/lightdm/io.elementary.greeter.conf \
       --replace "#default-wallpaper=/usr/share/backgrounds/elementaryos-default" \
-      "default-wallpaper=${nixos-artwork.wallpapers.simple-dark-gray}/share/artwork/gnome/nix-wallpaper-simple-dark-gray.png"
+      "default-wallpaper=${nixos-artwork.wallpapers.simple-dark-gray.gnomeFilePath}"
 
     substituteInPlace $out/share/xgreeters/io.elementary.greeter.desktop \
       --replace "Exec=io.elementary.greeter" "Exec=$out/bin/io.elementary.greeter"