E2E: Allow using separate github token for spec

What does this MR do?

Add ability to pass different github access token to test.

This is needed because test it self performs quite a lot of requests to github, when both app and test use the same token, it can hit api rate limit very quickly which will cause test to fail.

The test is only executed in specific project: https://gitlab.com/gitlab-org/manage/import/import-github-performance so there is no need to document this variable in scope of qa.