.cirrus.yml: bump FreeBSD to the current stable release

Hopefully this will un-stick the test which has been broken for a long
time.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Li-Wen Hsu <lwhsu@freebsd.org>
Tested-by: Li-Wen Hsu <lwhsu@freebsd.org>
Message-Id: <20200501111505.4225-4-alex.bennee@linaro.org>

.cirrus.yml

@@ -3,7 +3,7 @@ env:
 
 freebsd_12_task:
   freebsd_instance:
-    image: freebsd-12-0-release-amd64
+    image_family: freebsd-12-1
     cpu: 8
     memory: 8G
   install_script: pkg install -y