Revert "cirrus-ci: disable the FreeBSD 13 builds"

This reverts commit 691b71be93.

.cirrus.yml

@@ -6,7 +6,7 @@ task:
   freebsd_instance:
     matrix:
       # A stable 13.0 image likely won't be available before early 2021
-      # image_family: freebsd-13-0-snap
+      image_family: freebsd-13-0-snap
       image_family: freebsd-12-1
       # The stable 11.3 image causes "Agent is not responding" so use a snapshot
       image_family: freebsd-11-3-snap