Fix sync-examples workflow

[bluwy]

Changes

• Added checkout-ref input so we can sync the examples via a specific commit (we can use the commit before we merge next into main)
• Updated how we detect prereleases to check against .changeset/pre.json instead of the branch name
  • The bug before assumed that anything on main was stable which isn't always the case

---

There's two ways we can do after this is resolved:

1. Merge this PR, trigger the workflow manually to sync with the commit before we merged next to main, revert chore: downgrade examples to not use beta releases #12557, and it'll auto sync main to the beta branch instead.
2. Or disable this workflow, merge this PR, and re-enable the workflow on Tuesday again after we revert chore: downgrade examples to not use beta releases #12557 and prep the release

To prevent more work and to play it safe, I think no2 is the best for now.

Testing

n/a. Reading the code I think it'll work later.

Docs

n/a

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Latest commit: 2e1b602

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[bluwy]

We technically no longer need the github.ref == 'refs/heads/main' part, but I think it's better to leave that in to prevent cases where we create the next branch again but haven't entered prerelease, and we don't want to accidentally sync it to the latest branch examples.