E2E test: log in after creating mr in fork spec

What does this MR do and why?

Resolves https://gitlab.com/gitlab-org/gitlab/-/issues/472839

This spec is failing in canary only.

See https://gitlab.com/gitlab-org/gitlab/-/issues/472839#note_2003954761 for more context

How to set up and validate locally

bundle exec rspec qa/specs/features/browser_ui/3_create/merge_request/merge_merge_request_from_fork_spec.rb

Also tested against staging