Allow personal access token env var from gitlab-qa

What does this MR do?

gitlab-qa accepts an env var named GITLAB_QA_ACCESS_TOKEN, which is used to retrieve the version that staging is currently running when running the Test::Instance::Staging scenario.

The E2E tests here use PERSONAL_ACCESS_TOKEN to allow a personal access token to be specified for API access.

That means we couldn't set a personal access token for API access for the tests via gitlab-qa, at least not without adding another env var to gitlab-qa.

Instead of changing gitlab-qa, this allows either GITLAB_QA_ACCESS_TOKEN or PERSONAL_ACCESS_TOKEN to be used by the tests via Runtime::Env.personal_access_token.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated: gitlab-qa!213 (merged)
• 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