[CI] set token permissions for pre-commit CI job

[russellb]

Potential fix for https://github.com/vllm-project/vllm/security/code-scanning/21

To fix the issue, we will add a permissions block at the root level of the workflow file. This block will specify the least privileges required for the workflow to function correctly. Since the workflow only runs pre-commit checks and does not modify repository contents or interact with pull requests, it only requires contents: read permissions. This change will ensure that the workflow adheres to the principle of least privilege.

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[github-actions]

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