feat: add clean-cache workflow

[bigint]

• chore: update to use nightly env
• chore: update to use nightly env
• feat: add clean-cache workflow

What does this PR do?

🤖 Generated by Copilot at f0749e1

This pull request introduces two changes to the GitHub Actions workflows for the repository. It updates the nightly.yml workflow to use workflow-level environment variables and remove unnecessary code. It also adds a new clean-cache.yml workflow to manually clear the GitHub Actions cache.

Related issues

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Enhancement (non-breaking small changes to existing functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Explanation of the changes

🤖 Generated by Copilot at f0749e1

• Add a new workflow to manually clean the GitHub Actions cache for the repository (link)
• Add two environment variables to the nightly.yml workflow to indicate the CI and nightly build settings (link)
• Remove the redundant env block from the test step in the nightly.yml workflow (link)

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