Add GITHUB_USERNAME env var to resolver step

[neubig]

Fix the "Github username is required" error in the openhands-resolver workflow by adding the GITHUB_USERNAME environment variable to the resolver step, using the value from PAT_USERNAME secret.

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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=docker.all-hands.dev/all-hands-ai/runtime:44228ce-nikolaik   --name openhands-app-44228ce   docker.all-hands.dev/all-hands-ai/openhands:44228ce