Fix `NEXT_PUBLIC_` environment variables in cache

[taesungh]

Fixes #290.

Changes

• Replace NEXT_PUBLIC_VERCEL_URL with VERCEL_URL to avoid inlining
• Add NEXT_PUBLIC_SANITY_* to Turborepo pipeline.build.env cache key

Testing

Confirm deployment logs show that server-side API calls are going to the current deployment, even when the site build was from Full Turbo cache.

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