sources.json grows fast

[wamserma]

The sources.json file grows faster that anticipated (> 500kbytes/year). This will eventually increase checkout and evaluation time.

The initial goal was to keep this flake as lean and unobtrusive as possible, while also keeping information for as many channel updates as possible.

Solution to investigate:

• Do updates of sources.json to an archive branch. This still allows for using this flake with older revisions, but may break existing (older) configurations if this flake is not pinned in flake.lock.
• Extract only current versions to sources.json on the main branch. Filtering can be done on NixOS release numbers (as in the scraper) and sorting on the build numbers. The keep e.g. the latest 15 entries per active release.
• Filtering is done with a chained GitHub workflow.