Short-circuit next build before static generation starts

Implement a custom wrapper around `next build` which exits after the webpack build completes. This should make the analysis run much faster, and will generally simplify setups because the data fetching that happens during static generation can often require additional environment variables and/or secrets.

See: https://github.com/hashicorp/web-platform-packages/tree/main/packages/tools#next-build-webpack-only