build: integrate builtin_metadata.json handling in release process

[srenatus]

Fixes #4754.

Rule number one of release-script changes: it'll break the next release. 🤞

This is more convoluted than it has to be, but it's because of this outline of our release process:

• A clean checkout is used to generate the release patch,
  1. it'll update the version/version.go file
  2. it'll copy the capabilities files
• go isn't available in the docker container running that script (we're using python there)
• the golang script generate builtin_metadata.json consumes the capabilities files

So now, we'll bump the version in version/version.go and run make generate before handing stuff off to the python container with its release patch generation. With an extra conditional in the script generating the builtin_metadata.json when its version doesn't end in -dev, we'll add the currently-to-be-released version to each builtin's "available_versions".

This is a bit of a mess. If anyone has a better idea, I'm all ears. I'd eventually love to drop all our custom code in favor of something like ko, but that's for another day.

[ashutosh-narkar]

Have you tested this flow on your fork so we know what to expect when we create the next release. Also do you think we should add a note about this in RELEASE.md ?

[srenatus]

@ashutosh-narkar thanks, good points. Will do.

[srenatus]

✔️ Worked OK on my branch: https://github.com/srenatus/opa/actions/runs/2527597669