[CI] Add workflow permissions for helm CI job

[russellb]

Potential fix for https://github.com/vllm-project/vllm/security/code-scanning/24

To fix the issue, we will add a permissions block at the root level of the workflow. This block will specify the minimum permissions required for the workflow to function correctly. Based on the workflow's operations, it primarily reads repository contents and does not appear to require write permissions. Therefore, we will set contents: read as the default permission. If any specific steps require additional permissions, they can be added at the job level.

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[github-actions]

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