Use per-platform attrdiff data from GitHub Actions

[Zaczero]

Read attrdiffByPlatform from comparison artifacts and merge added/removed attrs into the imported per-system package set when that metadata is available.

Classify attrs with exists = false as non-existent before broken so platform-specific removals are reported accurately.

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Fixes issues like NixOS/nixpkgs#511376, NixOS/nixpkgs#507400, where removed packages show up as "No rebuilds" which is confusing and error-prone.

Instead, it shows:

--------- Report for 'x86_64-linux' ---------
2 packages present in ofBorgs evaluation, but not found in the checkout:
python313Packages.howdoi python314Packages.howdoi

The wording could be improved so that it explicitly distinguishes removed packages from simply missing packages, but that's a secondary thing.

Depends on new attrdiffByPlatform CI artifacts introduced in NixOS/nixpkgs#509519.