CI Failures from forks

Folks sending in a PR from their local forks will always cause a failure of the `ember-cli-update` CI job. This is due to the usage of `${{ secrets.GitHubToken }}` here:

https://github.com/ember-cli/ember-cli-update/blob/fc221be115e87049427d33f17cf11879637ea9c7/.github/workflows/ci.yml#L72-L73

Pull requests from forks will not have access to any `${{ secrets.* }}` content (for somewhat clear security reasons).

See [documentation here](https://docs.github.com/en/free-pro-team@latest/actions/reference/encrypted-secrets#using-encrypted-secrets-in-a-workflow), which starts out with:

> With the exception of GITHUB_TOKEN, secrets are not passed to the runner when a workflow is triggered from a forked repository.