startlxqtwayland: Ensure XDG_SESSION_DESKTOP is set

[Conan-Kudo]

This allows applications who read this variable to correctly identify the desktop.

[tsujan]

Here we need a colon-separated list, and we don't use XDG_SESSION_DESKTOP in startlxqt either.

This allows applications who read this variable to correctly identify the desktop.

They can do it with XDG_CURRENT_DESKTOP.

[tsujan]

Also, Arch Wiki says, "... Despite its name, it is not standardized by freedesktop.org."