ci: fix VKMS lookup after faux bus migration

VKMS has been migrated to the new faux bus. This causes breakage
in CI, because we used the platform bus to find the right device.

udev hasn't been updated yet to support the faux bus, so just use
sysfs instead.

(cherry picked from commit 03e7966650)

.builds/archlinux.yml

@@ -41,9 +41,10 @@ tasks:
       cd wlroots/build-gcc/tinywl
       sudo modprobe vkms
       udevadm settle
+      card="/dev/dri/$(ls /sys/devices/faux/vkms/drm/ | grep ^card)"
       export WLR_BACKENDS=drm
       export WLR_RENDERER=pixman
-      export WLR_DRM_DEVICES=/dev/dri/by-path/platform-vkms-card
+      export WLR_DRM_DEVICES="$card"
       export UBSAN_OPTIONS=halt_on_error=1
-      sudo chmod ugo+rw /dev/dri/by-path/platform-vkms-card
+      sudo chmod ugo+rw "$card"
       sudo -E seatd-launch -- ./tinywl -s 'kill $PPID' || [ $? = 143 ]