fix(CI): Don't require the GRAPHITE_TOKEN secret, as it's not available in forks (#69517)
> With the exception of `GITHUB_TOKEN`, secrets are not passed to the
runner when a workflow is triggered from a forked repository.
--
https://docs.github.com/en/actions/security-for-github-actions/security-guides/using-secrets-in-github-actions#using-secrets-in-a-workflow
## Test plan
Make a PR from a fork: #69518
Check the actions log and make sure it behaves as expected (fails open,
runs all tests):
<img width="1121" alt="Screenshot 2024-08-30 at 3 46 27 PM"
src="https://github.com/user-attachments/assets/e9ba7629-61da-431f-9822-8833dbb497b3">
<img width="309" alt="Screenshot 2024-08-30 at 3 46 35 PM"
src="https://github.com/user-attachments/assets/4d7d4cb0-ce2e-4cdd-bbdb-90de1347a77b">