Steps: update releases.json

[shawnlaffan]

It would be useful to update the releases.json file as part of the build process.

Doing so will simplify updates to strawberryperl.com, e.g. StrawberryPerl/strawberryperl.com#47

The url fields can be flagged as needing updates as they are usually done later.

[shawnlaffan]

Thinking further, it would be better to generate a subset of data which can then be merged with the main releases.json file.

This could be done either as its own step, or as a final part of the relevant build steps (MSI, portable, relocatable etc).

The system already stores the SHA1 for each file, for example.
https://github.com/search?q=repo%3AStrawberryPerl%2FPerl-Dist-Strawberry+sha1%7D&type=code