x11/plasma5-plasma-workspace: Fix XDG_CONFIG_DIRS for Wayland

Since Plasma 5.22, setupPlasmaEnvironment adds to XDG_CONFIG_DIRS, with
a fallback for the XDG-mandated /etc/xdg default. When using X11, this
is ok, as runStartupConfig gets called before setupPlasmaEnvironment,
and so the latter uses our modified value. However, when using Wayland,
they are called in the opposite order, and so runStartupConfig sees the
variable is already set and doesn't inject our modified default. Fix
this by changing setupPlasmaEnvironment's default value instead so it's
only set in one place. This approach also matches how other ports get
patched for our non-conforming environment.

Without this, the splash screen shows but plasmashell is never started
(nor any other autostart programs), eventually timing out and fading to
a black screen.

PR: 265505
Obtained from: CheriBSD
Approved by: maintainer timeout (3 months)