chore(ci): restore nucleus downstream testing

[wjhsf]

Details

The .nucleus.yaml file was removed in #5464 after we moved publishing fron Nucleus to GitHub Actions. We still need Nucleus for downstream testing, though, so this PR restores the file so we hopefully have the correct configuration. (I'm not 100% sure this is right, the docs aren't super clear.)

If you care about the diff, the first commit in this PR restores the file, then changes are made in subsequent commits.

Does this pull request introduce a breaking change?

• 😮‍💨 No, it does not introduce a breaking change.
• 💔 Yes, it does introduce a breaking change.

Does this pull request introduce an observable change?

• 🤞 No, it does not introduce an observable change.
• 🔬 Yes, it does include an observable change.

GUS work item

[rax-it]

Approving it as stopgap until we have a path forward with SFCI.

[wjhsf]

/nucleus ignore

[salesforce-nucleus]

❌ Error Parsing Command

Missing required argument: reason

/nucleus ignore

Ignore test failures

Options:
  --help        Show help                                              [boolean]
  --reason, -m  A descriptive reason for ignoring the test results
                                                             [string] [required]

Examples:
  /nucleus ignore --reason "Downstream dependency X has a flapping test"

Missing required argument: reason

[wjhsf]

/nucleus ignore --reason meh