Fix impersonation token e2e spec

What does this MR do and why?

Fixes browser_ui/1_manage/user/impersonation_token_spec.rb on live environments where the GITLAB_QA_ACCESS_TOKEN is different that GITLAB_QA_ADMIN_ACCESS_TOKEN and the former was being used on reload.

This MR ensures we use the admin api client (and so GITLAB_QA_ADMIN_ACCESS_TOKEN) for fetching the user's impersonation token which is only available to admins.

Fixes #369601 (closed)

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