ci: do not add ok-to-test if CentOS jobs were successful

Mergify does not add the `ok-to-test` label immediately anymore. But
once the CentOS CI jobs have finished, the label still gets added. With
the additional check, this should not be the case anymore.

Signed-off-by: Niels de Vos <ndevos@ibm.com>

.mergify.yml

@@ -52,6 +52,8 @@ pull_request_rules:
       - "check-pending=Queue: Embarked in merge train"
       - not:
           check-pending~=^ci/centos
+      - not:
+          status-success~=^ci/centos
     actions:
       label:
         add: