Ensure vendor gems child pipeline runs with gitlab-org runners

What does this MR do and why?

We must define runner tag in vendor gem child pipeline to make sure it uses the gitlab-org runners.

Re: Investigate sidekiq 7.1.6 vendored gem spec fla... (#460842 - closed)

Context: from Slack thread:

As pointed at https://handbook.gitlab.com/handbook/engineering/gitlab-repositories/#cicd-configuration. We have a finance analysis framework built around our runners and using public runners for gitlab-org or gitlab-com workloads makes the data improper

We can see that the sidekiq gem spec job is running with

Running with gitlab-runner 17.0.0~pre.88.g761ae5dd (761ae5dd)
  on green-1.shared-gitlab-org.runners-manager.gitlab.com/default Mf8beF5G, system ID: s_673deea48c2a

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