Fix broken master api_variables test

What does this MR do?

It looks like CI_API_V3_URL is not necessary for CE or EE, so I'm excluding the v3 version as a quick, dumb, simple fix for broken master.

This still allows for the original intent of the test to catch CI_API_V5_URL when that becomes a thing.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/9048

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer