Only select Sparkle items for macOS.

[reitermarkus]

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Apparently Sparkle appcasts can contain releases for other platforms, we only use this strategy for casks as far as I'm aware of, so exclude other platforms if the os attribute is specified.

Fixes CI failure in Homebrew/homebrew-cask#97558.

[BrewTestBot]

Review period will end on 2021-01-12 at 00:00:00 UTC.

[BrewTestBot]

Review period ended.