CI - Fix the external contributor check

[bfops]

Description of Changes

This was using a GH token that was not passed to external CI, so the contributor check would always fail. This was originally broken in https://github.com/clockworklabs/SpacetimeDB/pull/194.

I just switched it to using the GITHUB_TOKEN secret, which is passed to CI on external PRs (see https://docs.github.com/en/actions/security-for-github-actions/security-guides/using-secrets-in-github-actions#using-secrets-in-a-workflow).

We will also have to update the GITHUB_TOKEN token to have permissions to list org members.

API and ABI breaking changes

None. CI-only change.

Expected complexity level and risk

1

Testing

None, because I don't want to create a whole new GitHub account and repo just to open a test PR.

I say we merge this and test with one of the many open PRs, e.g. https://github.com/clockworklabs/SpacetimeDB/pull/2416 (where the author has signed the CLA).