Fix unsupported Vally workflow permission

[Copilot]

The actionlint job failed because vally-eval.yml declared copilot-requests, which is not a recognized GitHub Actions permission scope.

• Workflow permissions
  • Remove copilot-requests: write from the Vally evaluation workflow.
  • Retain the required repository and pull-request read permissions.

permissions:
  contents: read
  pull-requests: read

[azure-pipelines]

Azure Pipelines:
Successfully started running 1 pipeline(s).
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