Add Cirrus CI -Wformat=2 checks to Rocky Linux.

We lost level 2 when we swapped the sanitizers between ubuntu gcc and
clang, as Clang's -Wformat=2 is too spammy and we couldn't keep it on
gcc as it breaks the sanitizers.  So this puts it onto a third build
instead.

.cirrus.yml

@@ -141,7 +141,7 @@ rocky_task:
     LC_ALL: C
     CIRRUS_CLONE_DEPTH: 1
     USE_CONFIG: yes
-    CFLAGS: -std=gnu90
+    CFLAGS: -std=gnu90 -Wformat -Wformat=2
 
   # NB: we could consider building a docker image with these
   # preinstalled and specifying that instead, to speed up testing.