Fixed bug #6, explicitly set the QT platform for the shell

#6

When a user with QT_QPA_PLATFORM=wayland in their environment runs this application, it crashes with qt.qpa.plugin: Could not find the Qt platform plugin "wayland" in "". It might be a good idea to set this environment variable to xcb as in https://github.com/flathub/org.keepassxc.KeePassXC/pull/71/files.

com.valvesoftware.SteamLink.yml

@@ -8,6 +8,7 @@ finish_args:
   - --device=all
   - --env=LD_LIBRARY_PATH=/app/lib
   - "--env=SDL_GAMECONTROLLERCONFIG_FILE=/var/data/Valve Corporation/SteamLink/controller_map.txt"
+  - --env=QT_QPA_PLATFORM=xcb
   - --share=ipc
   - --share=network
   - --socket=pulseaudio