CloudFlare build-system v3 compatibility

[philpennock]

Our CloudFlare deployment has some static files, not served by special handlers, and we use ./cdn-site-build as the entry-point for that (as documented in CLOUDFLARE.md).

CloudFlare have deprecated v1 of their build-system, and v2 too. We need to move to v3 (or move off CloudFlare). We used v1 as it was current when this was written.

Releases after v1 no longer include jq, which breaks the make-json-files used from cdn-site-build.

Minimally:

• rewrite make-json-files to Python
  • Per https://developers.cloudflare.com/pages/configuration/build-image/ Python is still included
  • I ignored Python at the time because it was stuck on v2.7, but build image v2 onwards include Python 3 instead
  • Image v3 includes Python 3.13.3 by default (and others can be selected)