retropiemenu: fix launching RetroArch from the menu

Due to the recent addition of the GameMode support, RetroArch needs access to the user's DBUS session socket.
When starting RetroArch from the RetroPie menu, `su` is used to launch it as the install user, but the DBUS socket path is not preserved (`DBUS_SESSION_BUS_ADDRESS`), leading to a en error and then a crash.

Prevent the crash by setting the XDG_RUNTIME_DIR when RetroArch is started with `su`.

NOTE: this primarily affects X11/PC users, where GameMode is part of the desktop install.

Author: cmitu

scriptmodules/supplementary/retropiemenu.sh

@@ -138,7 +138,7 @@ function launch_retropiemenu() {
             joy2keyStop
             cp "$configdir/all/retroarch.cfg" "$configdir/all/retroarch.cfg.bak"
             chown $user:$user "$configdir/all/retroarch.cfg.bak"
-            su $user -c "\"$emudir/retroarch/bin/retroarch\" --menu --config \"$configdir/all/retroarch.cfg\""
+            su $user -c "XDG_RUNTIME_DIR=/run/user/$SUDO_UID \"$emudir/retroarch/bin/retroarch\" --menu --config \"$configdir/all/retroarch.cfg\""
             iniConfig " = " '"' "$configdir/all/retroarch.cfg"
             iniSet "config_save_on_exit" "false"
             ;;