ayatana-indicator-messages: Fix desktop parsing

Not *great*, but a start.

MessagingMenu has a function for mapping an app to desktop file data. This relies on GLib to parse the desktop file,
which needs the binary from the 'Exec=' entry on PATH, otherwise it bails.

messaging_menu_app_set_desktop_id calls g_desktop_app_info_new:
334668ad5a/libmessaging-menu/messaging-menu-app.c (L245-L252)

Which, after some jumps inbetween, ends up in g_desktop_app_info_load_from_keyfile and calls g_find_program_for_path:
204a76669b/gio/gdesktopappinfo.c (L1920-L1951)

Which uses PATH:
204a76669b/glib/gutils.c (L371)

pkgs/by-name/ay/ayatana-indicator-messages/package.nix

@@ -119,6 +119,15 @@ stdenv.mkDerivation (finalAttrs: {
     chmod +w doc/reference/html/*
   '';
 
+  # Indicators that talk to it may issue requests to parse desktop files, which needs binaries in Exec on PATH
+  # messaging_menu_app_set_desktop_id -> g_desktop_app_info_new -...-> g_desktop_app_info_load_from_keyfile -> g_find_program_for_path
+  # When launched via systemd, PATH is very narrow
+  preFixup = ''
+    gappsWrapperArgs+=(
+      --suffix PATH : '/run/current-system/sw/bin'
+    )
+  '';
+
   passthru = {
     ayatana-indicators = [
       "ayatana-indicator-messages"