Revert "guix: node-build-system: Use guile-json instead of a custom parser."

Revert "guix: node-build-system: Use guile-json instead of a custom parser."

The effect of this change was to import the (json parser) from the host
side into the build side. The solution here would be to do the equivalent
of ‘with-extensions’ for gexps. Since we don't use gexps for build
systems just yet, revert this for now.

This reverts commit 8eb0ba532ebbebef23180e666e0607ea735f9c1a.