ci: update freebsd 12 cirrus job

make sure it uses a supported OS branch and uses all the resources
that can be allocated efficiently.

while only 1GB of memory is needed, 2GB is the minimum for a 2 CPU
machine (the default), but by increasing parallelism wall time has
been reduced by 35%.

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.cirrus.yml

@@ -2,8 +2,15 @@ env:
   CIRRUS_CLONE_DEPTH: 1
 
 freebsd_12_task:
+  env:
+    GIT_PROVE_OPTS: "--timer --jobs 10"
+    GIT_TEST_OPTS: "--no-chain-lint --no-bin-wrappers"
+    MAKEFLAGS: "-j4"
+    DEFAULT_TEST_TARGET: prove
+    DEVELOPER: 1
   freebsd_instance:
-    image: freebsd-12-1-release-amd64
+    image_family: freebsd-12-2
+    memory: 2G
   install_script:
     pkg install -y gettext gmake perl5
   create_user_script: