preferences: Indicate that option is overridden by ENV

[Rafostar]

With some mobile distros/tools exporting experimental ENV options by default without user knowledge (example, also others like PureOS may set USE_PLAYBIN3), we really need to indicate such behavior inside player prefs and maybe print a warning at startup.

Otherwise users of these distros will be affected by playbin3 bugs (the env will also affect Flatpak version which does not even need it) and will likely come complaining here about something not working correctly without knowing that they have enabled experimental features.