kscreenlocker nvidia crash

Seems there are problems with the latest sandboxing done in the kscreenlocker application. When locking the users only sees a message about using `loginctl unlock-sessions` to unlock their session. They then have to use a terminal to run that command to unlock their session.

Running `/usr/lib/x86_64-linux-gnu/libexec/kscreenlocker_greet --testing` triggers a segfault.

I'm using nvidia-304 for a legacy card but the upstream reports are against the latest driver:

https://devtalk.nvidia.com/default/topic/1023193/384-69-broke-kde-screen-locker-possibly-other-qt-based-software-on-linux/

https://bugs.kde.org/show_bug.cgi?id=384005