Build the Xamarin.PreBuilt.iOS app bundle instead of using a prebuilt bundle.
This makes sure that we're always using the latest BCL.
Some accurate build massaging was needed, because:
* To build the prebuilt app we need the iOS workload installed (into our build-local
.NET installation).
* The iOS workload contains the Microsoft.iOS.Windows.Sdk pack.
* The Microsoft.iOS.Windows.Sdk pack contains the prebuilt app.
Thus we had a circular reference. Fortunately, the Microsoft.iOS.Windows.Sdk pack
is only required on Windows, which means we can break this circular reference by:
* Mark Microsoft.iOS.Windows.Sdk pack as only to be installed on Windows (unfortunately
it seems we have to list the exact runtime identifiers for the platforms where
to install the pack, so we can't do something like "win-*", but new variations
of the "win-*" runtime identifier shouldn't show up all that often).
* Build the prebuilt app on macOS.
This way we don't need the Microsoft.iOS.Windows.Sdk pack when installing the iOS
workload locally.
The .NET build order is now:
* Build general sdk, runtime and ref packs for .NET.
* Build the prebuilt app.
* Build the Microsoft.iOS.Windows.Sdk pack.
Fixes https://github.com/xamarin/xamarin-macios/issues/12945.
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