[CI] Add token permissions for add-ready-label CI job

[russellb]

Potential fix for https://github.com/vllm-project/vllm/security/code-scanning/20

To fix the issue, we will add a permissions block at the workflow level to explicitly define the least privileges required for the workflow. Since the workflow uses the issues.addLabels API, it requires issues: write permission. All other permissions will be set to read or omitted entirely.

The permissions block will be added at the root level of the workflow file, ensuring it applies to all jobs in the workflow.

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[github-actions]

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