Remove ambiguity in Group class causing build failures

https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/71887977 was failing
due to confusion between Ci::Group and Group.
https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/71887977

app/services/ci/register_job_service.rb

@@ -90,7 +90,7 @@ module Ci
     def builds_for_group_runner
       # Workaround for weird Rails bug, that makes `runner.groups.to_sql` to return `runner_id = NULL`
-      groups = Group.joins(:runner_namespaces).merge(runner.runner_namespaces)
+      groups = ::Group.joins(:runner_namespaces).merge(runner.runner_namespaces)
       hierarchy_groups = Gitlab::GroupHierarchy.new(groups).base_and_descendants
       projects = Project.where(namespace_id: hierarchy_groups)