dont run pr update on forks

[rbren]

End-user friendly description of the problem this fixes or functionality that this introduces

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below
  no changelog

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Give a summary of what the PR does, explaining any non-trivial design decisions

CI is failing on forked PRs because of this

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Link of any specific issues this addresses

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=ghcr.io/all-hands-ai/runtime:5db76b5-nikolaik   --name openhands-app-5db76b5   ghcr.io/all-hands-ai/runtime:5db76b5