Update build.gradle to support nightly reference tests

[jtraglia]

PR Description

Recently, the consensus-specs repo started generating nightly reference tests:

• https://github.com/ethereum/consensus-specs/actions/workflows/generate_vectors.yml

To test this, do the following:

• Unfortunately, we need a token to download artifacts programmatically.
  • 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)
  • 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.
• Download/expand/run the nightly reference tests like so:
  • NIGHTLY=true ./gradlew expandRefTests
  • ./gradlew referenceTest

Output from expandRefTests will look like:

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.