Platform reports iOS while running on macOS

[AvdLee]

See this line of code: https://github.com/amplitude/experiment-ios-client/blob/main/Sources/Experiment/DefaultUserProvider.swift#L54

When using the SDK on macOS, I would expect this value to be macOS.
The Amplitude SDK seems to do this differently, since our analytics still show users for macOS. However, variants are fetched using iOS instead:

[Experiment] Fetch variants for user: ["platform": "iOS", "version": "2.0.0", "user_id": "XXX", "language": "English", "os": "ios 13.3.1.", "device_id": "XXX", "device_manufacturer": "Apple", "device_model": "arm64", "library": "experiment-ios-client/1.9.0"]

Note the value for os being ios 13.3.1.: the macOS version is correct, but it's prefixed with iOS instead of macOS. Due to this, our experiment didn't work as we were targeting macOS users.

[bgiori]

Thanks for this report @AvdLee.

I'll add a ticket for updating this currently hardcoded field