run: Normalize WAYLAND_DISPLAY to a reasonable name
run: Normalize WAYLAND_DISPLAY to a reasonable name

If it doesn't start with wayland- or contains a directory separator,
then it's probably not something we want to be creating in the
sandbox's XDG_RUNTIME_DIR.

Signed-off-by: Simon McVittie <0e1aea601907c1a85c2dc96e5ea37e23c58d2e9b@collabora.com>

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak