ci: Retry submodule sync 2 more times before failing

.gitlab-ci.yml

@@ -391,6 +391,7 @@ check_submodule_sync:
   dependencies: []
   variables:
     GIT_STRATEGY: clone
+    retry: 2
   before_script: *do_nothing_before
   script:
     # check if all submodules are correctly synced to public repostory