Add 1 instance runner to the runner fleet rake seed

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR builds up on the recently merged rake task to make one of the runners generated be an instance-level runner.

Screenshots or screen recordings

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before	after

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Run the following command:

bundle exec rake "gitlab:seed:runner_fleet[root, rf-]"

You can use tail -f log/application_json.log to follow the operation logs.

You should then be able to see the 1 instance runner in the instance runners tab.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.