cosmic-bg: add FHS substitutes and upstream patch (#398312)

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

@@ -3,6 +3,8 @@
   stdenv,
   fetchFromGitHub,
   rustPlatform,
+  fetchpatch,
+  cosmic-wallpapers,
   libcosmicAppHook,
   just,
   nasm,
@@ -21,6 +23,20 @@ rustPlatform.buildRustPackage (finalAttrs: {
     hash = "sha256-4b4laUXTnAbdngLVh8/dD144m9QrGReSEjRZoNR6Iks=";
   };
 
+  patches = [
+    # TOOD: This is merged and will be included in the 7th Alpha release, remove it then.
+    (fetchpatch {
+      url = "https://github.com/pop-os/cosmic-bg/commit/6a824a7902d7cc72b5a3117b6486603a1795a1d6.patch";
+      hash = "sha256-jL0az87BlJU99lDF3jnE74I4m/NV6NViyYXTfZoBDM4=";
+    })
+  ];
+
+  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=";