Fix CI for external contributors

[mrmr1993]

This PR

• adds a field to buildkite requests in the ci-build-me tool so that CI jobs run for forked pull requests
  • providing the repository field allows buildkite to find the commit and branch in the forked repo so that it can test it
  • this was tested manually on Updated Rosetta API to encode random oracle input as hex string #6703 (triggered CI run here)
• changes the branch passed to CircleCI from the branch name to the special pull/PR_NUMBER/head branch
  • this was also tested manually on Updated Rosetta API to encode random oracle input as hex string #6703 (triggered CI run here)

Checklist:

• Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• All tests pass (CI will check this if you didn't)
• Serialized types are in stable-versioned modules
• Does this close issues? List them:

[mrmr1993]

Updated to improve logging.

After debugging, it turns out that the user making the comment needs to publically be a member of the org. At present, this means that only @bkase and I can trigger CI jobs with the !ci-build-me comment.

[mrmr1993]

Merging, this has been deployed for a week working correctly and without issue.