xwfb-run errors out when run without XDG_RUNTIME_DIR

Hi, I was trying to replace xvfb-run with xwfb-run in one of my CI scripts, but saw that it was failing.

The problem seems to be that each compositor (or at least mutter, cage and weston, but logically I think we can assume the rest of them too) relies on the XDG_RUNTIME_DIR environment variable to be set.

In a full graphical session it makes sense of course, but ideally we don't have to jump through many hoops to get this to run in some plain CI docker.

Note also that xvfb-run does not need it to be set, so it runs just fine.