qtractor: fix GTK3 GSettings schema lookup

pkgs/by-name/qt/qtractor/package.nix

@@ -6,6 +6,7 @@
   dssi,
   fetchurl,
   flac,
+  gtk3,
   libjack2,
   ladspa-header,
   ladspaPlugins,
@@ -45,6 +46,16 @@ stdenv.mkDerivation (finalAttrs: {
     qt6.wrapQtAppsHook
   ];
 
+  # Qt's GTK3 file chooser uses GSettings. GTK3's GSettings schemas
+  # are installed below share/gsettings-schemas, which is not otherwise
+  # exposed to the wrapped Qt application.
+  qtWrapperArgs = [
+    "--suffix"
+    "XDG_DATA_DIRS"
+    ":"
+    "${gtk3}/share/gsettings-schemas/${gtk3.name}"
+  ];
+
   buildInputs = [
     alsa-lib
     aubio