cosmic-bg: substitute default background path

The directory `/usr/share/backgrounds/cosmic/` is not accessible on
NixOS because the directory `/usr` does not exist on NixOS. But the
wallpaper image that upstream uses as the default wallpapers is packaged
in the `cosmic-wallpapers` package. So substitute the FHS path with the
path of the wallpaper image from the Nix store, making this work on
NixOS _and_ non-NixOS operating systems.

pkgs/by-name/co/cosmic-bg/package.nix

@@ -3,6 +3,7 @@
   stdenv,
   fetchFromGitHub,
   rustPlatform,
+  cosmic-wallpapers,
   libcosmicAppHook,
   just,
   nasm,
@@ -21,6 +22,12 @@ rustPlatform.buildRustPackage (finalAttrs: {
     hash = "sha256-4b4laUXTnAbdngLVh8/dD144m9QrGReSEjRZoNR6Iks=";
   };
 
+  postPatch = ''
+    substituteInPlace config/src/lib.rs data/v1/all \
+      --replace-fail '/usr/share/backgrounds/cosmic/orion_nebula_nasa_heic0601a.jpg' \
+      "${cosmic-wallpapers}/share/backgrounds/cosmic/orion_nebula_nasa_heic0601a.jpg"
+  '';
+
   useFetchCargoVendor = true;
   cargoHash = "sha256-GLXooTjcGq4MsBNnlpHBBUJGNs5UjKMQJGJuj9UO2wk=";