cosmic-workspaces-epoch: substitute fallback background path

The directory `/usr/share/backgrounds/pop/` is not accessible on NixOS
because the directory `/usr` does not exist on NixOS. The package
`cosmic-wallpapers` exists in nixpkgs that provides the background image
that is used in the COSMIC DE's screenshots on the first-party
website[0]. Substitute the unreachable, FHS path with a valid path of
the background image from the Nix store.

Note: We are putting in a different fallback background path alltogether
but I believe that this is okay because that is used by upstream in
their marketing materials. Meaning, after substitution, we match
marketing screenshots from upstream.

[0]: https://system76.com/cosmic/

pkgs/by-name/co/cosmic-workspaces-epoch/package.nix

@@ -3,6 +3,7 @@
   stdenv,
   rustPlatform,
   fetchFromGitHub,
+  cosmic-wallpapers,
   libcosmicAppHook,
   pkg-config,
   libinput,
@@ -23,6 +24,22 @@ rustPlatform.buildRustPackage (finalAttrs: {
     hash = "sha256-3jivE0EaSddPxMYn9DDaYUMafPf60XeCwVeQegbt++c=";
   };
 
+  postPatch = ''
+    # While the `kate-hazen-COSMIC-desktop-wallpaper.png` image is present
+    # in the `pop-wallpapers` package, we're using the Orion Nebula image
+    # from NASA available in the `cosmic-wallpapers` package. Mainly because
+    # the previous image was used in the GNOME shell extension and the
+    # Orion Nebula image is widely used in the Rust-based COSMIC DE's
+    # marketing materials. Another reason to use the Orion Nebula image
+    # is that it's actually the default wallpaper as configured by the
+    # `cosmic-bg` package's configuration in upstream [1] [2].
+    #
+    # [1]: https://github.com/pop-os/cosmic-bg/blob/epoch-1.0.0-alpha.6/config/src/lib.rs#L142
+    # [2]: https://github.com/pop-os/cosmic-bg/blob/epoch-1.0.0-alpha.6/data/v1/all#L3
+    substituteInPlace src/view/mod.rs \
+      --replace-fail '/usr/share/backgrounds/pop/kate-hazen-COSMIC-desktop-wallpaper.png' "${cosmic-wallpapers}/share/backgrounds/cosmic/orion_nebula_nasa_heic0601a.jpg"
+  '';
+
   useFetchCargoVendor = true;
   cargoHash = "sha256-l5y9bOG/h24EfiAFfVKjtzYCzjxU2TI8wh6HBUwoVcE=";