Enable CKI on os-build MRs only

Not all the old Fedora branches have the latest Makefile and specfile
changes to support the self-test check.  Disable the Fedora branches for
now.

Signed-off-by: Don Zickus <dzickus@redhat.com>

.gitlab-ci.yml

@@ -56,6 +56,8 @@ workflow:
     skip_publish: 'true'
     skip_test: 'true'
     skip_results: 'true'
+  rules:
+    - if: '$CI_MERGE_REQUEST_TARGET_BRANCH_NAME == "os-build"'
 
 .9-common:
   variables: