x11/plasma5-plasma-workspace: setup XDG_CONFIG_DIRS on startup if not set

In prior versions of KDE's Plasma Desktop we set XDG_CONFIG_DIRS in the startkde
shellscript. Plasma 5.17 changed to a binary for this process and that variable
therefore was no longer set for most people (unless manually done so in their
environment / .xinitrc file, like me...).

With this patch the variable is now again setup, startplasma should be able to
bring up the whole shell again.

PR: 241341
Reported by: Martin Birgmeier