[autotest/wayland] Force kscreenlocker_greet to QMLSCENE_DEVICE softwarecontext

OpenGL/EGL doesn't work on build.kde.org. Because of that the
kscreenlocker_greet aborts on startup and we cannot really test whether
it works on build.kde.org. E.g. we never actually see the window.

This change just makes sure that QMLSCENE_DEVICE=softwarecontext is
passed to the greeter in the LockScreenTest. Hopefully that fixes it
on build.kde.org.