CODEOWNERs verification workflow fails when run on PR from forked repo

GitHub actions running on PRs from forked only have access to the _source_ repo's secrets, not the target's. The verification workflow needs our API token. This is a known issue: https://github.community/t/make-secrets-available-to-builds-of-forks/16166

We can workaround by checking for that secret and only running the owners check if it's found.