cinnamon.nemo: add missing xapp icons

Nemo is used outside cinnamon (e.g. budgie) these days.
Only cinnamon is shipping xapp as environment.systemPackages.

We probably should do this to (mostly) all mint apps but probably
when users wish to use them outside cinnamon.

pkgs/desktops/cinnamon/nemo/default.nix

@@ -67,6 +67,13 @@ stdenv.mkDerivation rec {
     "--localedir=${cinnamon-translations}/share/locale"
   ];
 
+  preFixup = ''
+    # Used for some non-fd.o icons (e.g. xapp-text-case-symbolic)
+    gappsWrapperArgs+=(
+      --prefix XDG_DATA_DIRS : "${xapp}/share"
+    )
+  '';
+
   # Taken from libnemo-extension.pc.
   passthru.extensiondir = "lib/nemo/extensions-3.0";