cosmic-settings-daemon: substitute generic "Pop theme" path

The directory `/usr/share/sounds/Pop/` is not accessible on NixOS
because the directory `/usr` does not exist on NixOS. The package
`pop-gtk-theme` exists in nixpkgs that provides this exact "theme
sound" which upstream uses. Substitute the FHS path with the path of the
`pop-gtk-theme` package from the Nix store.

pkgs/by-name/co/cosmic-settings-daemon/package.nix

@@ -3,6 +3,7 @@
   fetchFromGitHub,
   stdenv,
   rustPlatform,
+  pop-gtk-theme,
   pkg-config,
   geoclue2-with-demo-agent,
   libinput,
@@ -21,6 +22,11 @@ rustPlatform.buildRustPackage (finalAttrs: {
     hash = "sha256-DtwW6RxHnNh87Xu0NCULfUsHNzYU9tHtFKE9HO3rvME=";
   };
 
+  postPatch = ''
+    substituteInPlace src/battery.rs \
+      --replace-fail '/usr/share/sounds/Pop/' '${pop-gtk-theme}/share/sounds/Pop/'
+  '';
+
   useFetchCargoVendor = true;
   cargoHash = "sha256-lGzQBL9IXbPsaKeVHp34xkm5FnTxWvfw4wg3El4LZdA=";