Add support for nightly reference tests

[jtraglia]

PR Description

This is still a work in progress.

• Create a GitHub access token at https://github.com/settings/tokens with the following perms:
  • repo, workflow, admin:repo_hook (not sure these are actually necessary, but it's what I did)
  • Unfortunately, we need a token to download artifacts programmatically.
  • I would recommend using a burner GitHub account for this just in-case it gets burned.
• Export this as an environment variable under GITHUB_TOKEN.
  • I just added a export GITHUB_TOKEN="ghp_..." line to my ~/.zshrc file.
• Run NIGHTLY=true ./gradlew expandRefTests

Next steps:

Add a scheduled CI test that runs at 6am UTC. This is 4 hours after the reference tests are generated. Normally, it takes about 3.5 hours for the test generators to finish.

Notes:

It would have been slightly easier to use gh (GitHub's CLI tool) in a CI test, but I think doing it with Gradle is the better approach for Teku. Easier to maintain and works better with the dev flow.

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.