Handle chromatic when run from forks

Currently, forks do not have a project token, and our [Chromatic job fails](https://github.com/guardian/dotcom-rendering/actions/runs/5476003095/jobs/9972903382) without helpful information. The status should probably be “skipped” rather than “fail”

It’s still possibly to [manually run the chromatic job](https://www.chromatic.com/docs/cli) by checking out the branch locally, and it will allow review from @guardian/dotcom-platform.