Fix downstream file variable spec to not create another pipeline

What does this MR do and why?

Attempting to address flakiness in https://gitlab.com/gitlab-org/gitlab/-/blob/master/qa/qa/specs/features/api/4_verify/file_variable_downstream_pipeline_spec.rb

Failure https://gitlab.com/gitlab-org/gitlab/-/issues/419474

What I observed from the latest failure report is that, there were 2 pipelines created in upstream project where we only expected 1. I can't really explain why that would cause both pipelines to failed which lead to the test timeout and the above failure. In this MR, I attempted to:

• Retrieve the upstream_pipeline by providing its id when call create. Because without id, a new pipeline would be created.
• Rename the variables and methods for readability.

I ran the test multiple times against GDK and Staging with this fix and it passed every time. Hopefully this is just what it needs.

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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.

To run against GDK

$ GITLAB_QA_ACCESS_TOKEN=<PAT> QA_GITLAB_URL=http://<local.ip.adress>:3000/ bundle exec rspec qa/specs/features/api/4_verify/file_variable_downstream_pipeline_spec.rb

To run against Staging

$ GITLAB_ADMIN_USERNAME="gitlab-qa-bot" GITLAB_ADMIN_PASSWORD=<password> GITLAB_QA_USER_AGENT=<agent> GITLAB_QA_ADMIN_ACCESS_TOKEN=<token> QA_GITLAB_URL=https://staging.gitlab.com bundle exec rspec qa/specs/features/api/4_verify/file_variable_downstream_pipeline_spec.rb

MR acceptance checklist

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

Related to #419474