Fix slack channel for packages release notification

[JayJay1024]

PR-Codex overview

This PR updates the environment variable naming convention in the .github/workflows/release.yml file, changing the slack_channel variable to SLACK_CHANNEL to follow a consistent uppercase format.

Detailed summary

• Changed slack_channel to SLACK_CHANNEL in the environment variables section of .github/workflows/release.yml.

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Latest commit: 5b2c07b

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