BUILD: cirrus-ci: switch to "snap" images to unify openssl naming

"snap" images are updated frequently, while regular images are updated quarterly.
so, mixing "snap" and regular images lead to package naming mismatch, which will occur every
quarter. we cannot use 11.3 release image, it is broken, so we have to use 11.3 "snap" image.
Thus let us use all "snap" images. 13-snap is first introduced with this commit.

.cirrus.yml

@@ -1,17 +1,12 @@
 FreeBSD_task:
   freebsd_instance:
     matrix:
-      image_family: freebsd-12-1
+      image_family: freebsd-13-0-snap
+      image_family: freebsd-12-1-snap
       image_family: freebsd-11-3-snap
   only_if: $CIRRUS_BRANCH =~ 'master|next'
   install_script:
-    - case `uname -r` in
-        11.3*)
-          export SSL=openssl;;
-        12.1*)
-          export SSL=openssl111;;
-      esac
-    - pkg install -y $SSL git gmake lua53 socat
+    - pkg install -y openssl git gmake lua53 socat
   script:
     - git clone https://github.com/VTest/VTest.git ../vtest
     - make -C ../vtest