Document the usage of MOZ_ENABLE_WAYLAND=0 in README to use systray-x under Wayland

[Antiz96]

Since version 128, Thunderbird uses Wayland by default. To get the systray-x addon working properly under Arch on Sway with Thunderbird 128, I had to use env MOZ_ENABLE_WAYLAND=0 thunderbird (instead of env GDK_BACKEND=x11 thunderbird, see this comment).

To be honest, it seems DE/Compositor specific (somehow). For instance, env GDK_BACKEND=x11 thunderbird still works in KDE, but doesn't seem to work anymore in Sway (with Thunderbird 128+ at least, it was working fine with previous version of Thunderbird though). Regardless, I think it doesn't hurt documenting env MOZ_ENABLE_WAYLAND=0 thunderbird in the README in case people face the same issue I got on Sway with Thunderbird 128+.

I also removed the AUR mention for the "Alternative install" in the Arch section as it is not about an AUR package but an official Arch package from the [extra] repository.