external RPC checks: workflow_dispatch against a branch/image and a label trigger

[EclesioMeloJunior]

Summary

Follow-up to #7477 (comment).

.github/workflows/external-rpc-checks.yml currently only runs on a daily schedule and a bare workflow_dispatch, always against ghcr.io/chainsafe/forest:edge-fat (hardcoded in scripts/tests/external-rpc-checks/docker-compose.yaml). There is no way to point it at a suspicious branch, nor to opt a PR into the check.

Two related capabilities are wanted:

1. Dispatch against a specific build. workflow_dispatch should accept an image (and ideally build one locally from the selected ref), so a branch can be exercised against the external dataset before merging. rpc-parity.yml already does the image-input

2. Label-driven runs on PRs, mirroring the existing RPC / Snapshot labels.

Completion Criteria

• The workflow can run against a branch, building the image from that ref if no image is given
• A PR label opts a pull request into the check, with a no-op job so the required check still reports

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