refactor(storybook): use STORYBOOK_* env variables

[joshblack]

It seems like in newer versions of storybook, only STORYBOOK_ prefixed environment variables are passed along. This PR updates our ENV variables to follow this standard and re-enable our theme preview in the classic storybook

Changelog

New

Changed

• Update classic storybook to use STORYBOOK_* prefixed ENV variables

Removed

Testing / Reviewing

• Verify theme panel shows up in our classic storybook and is able to change the theme

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[netlify]

✔️ Deploy Preview for carbon-components-react ready!
Built without sensitive environment variables

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