cosmic-osd: substitue path for audio-volume-change.oga file

The path `/usr/share/sounds/freedesktop/stereo/audio-volume-change.oga`
is not accessible on NixOS because the directory `/usr` does not exist
on NixOS. But the `audio-volume-change.oga` file is available in the
`sound-theme-freedesktop` package. So substitute the full, FHS-compliant
path with the path of the `sound-theme-freedesktop` package from the Nix
store.

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

@@ -1,6 +1,7 @@
 {
   lib,
   fetchFromGitHub,
+  sound-theme-freedesktop,
   rustPlatform,
   libcosmicAppHook,
   pulseaudio,
@@ -20,6 +21,11 @@ rustPlatform.buildRustPackage (finalAttrs: {
     hash = "sha256-ezOeRgqI/GOWFknUVZI7ZLEy1GYaBI+/An83HWKL6ho=";
   };
 
+  postPatch = ''
+    substituteInPlace src/components/app.rs \
+      --replace-fail '/usr/share/sounds/freedesktop/stereo/audio-volume-change.oga' '${sound-theme-freedesktop}/share/sounds/freedesktop/stereo/audio-volume-change.oga'
+  '';
+
   useFetchCargoVendor = true;
   cargoHash = "sha256-vYehF2RjPrTZiuGcRUe4XX3ftRo7f+SIoKizD/kOtR8=";