ci: halt on UBSan error in smoke test

By default UBSan prints a message and carries on, which makes it
easy to miss errors.

.builds/archlinux.yml

@@ -44,5 +44,6 @@ tasks:
       export WLR_BACKENDS=drm
       export WLR_RENDERER=pixman
       export WLR_DRM_DEVICES=/dev/dri/by-path/platform-vkms-card
+      export UBSAN_OPTIONS=halt_on_error=1
       sudo chmod ugo+rw /dev/dri/by-path/platform-vkms-card
       sudo -E seatd-launch -- ./tinywl -s 'kill $PPID' || [ $? = 143 ]