common: Create and export XDG_RUNTIME_DIR if needed

Most Wayland compositor rely on the environment variable XDG_RUNTIME_DIR to be set and the corresponding directory to be accessible.

If XDG_RUNTIME_DIR isn't set (as with some plain CI docker setups), create a temporary directory and set the environment variable accordingly.

Closes: #3 (closed) Signed-off-by: Olivier Fourdan ofourdan@redhat.com