ci: switch to seatd-launch

This removes the need for the ad-hoc loop.

Because udev creates the symlinks in /dev/dri/by-path/, we need to
wait for it to consume all pending events before the chmod call.
Previously the delay needed for seatd to come up was enough to let
udev create the symlinks in time (by chance).

.builds/archlinux.yml

@@ -34,10 +34,9 @@ tasks:
   - smoke-test: |
       cd wlroots/tinywl
       sudo modprobe vkms
-      sudo seatd -u "$USER" &
-      while ! [ -e /run/seatd.sock ]; do sleep 0.1; done
+      udevadm settle
       export WLR_BACKENDS=drm
       export WLR_RENDERER=pixman
       export WLR_DRM_DEVICES=/dev/dri/by-path/platform-vkms-card
       sudo chmod ugo+rw /dev/dri/by-path/platform-vkms-card
-      ./tinywl -s 'kill $PPID' || [ $? = 143 ]
+      sudo -E seatd-launch -- ./tinywl -s 'kill $PPID' || [ $? = 143 ]