(macOS pkg for) iOS workload doesn't include sentinel

[chucker]

(Related: #47)

I'm not sure if this is by design, but the Android workload adds a .sentinel file to ./sdk/, and the iOS one doesn't:

Consequently, only installing the SDK and the iOS workload will result in the "The target platform identifier ios was not recognized" error.

If this is intended, the README should make it clearer that building the iOS sample on macOS will fail unless you also install the Android workload.

[jonathanpeppers]

We are hoping the need for a .sentinel file at all can go away.

The dotnet/sdk did this so they could ship workload support behind a feature flag in .NET 5. I think they can get rid of it soon.

[jonathanpeppers]

In .NET 6 Preview 4, EnableWorkloadResolver.sentinel is no longer needed.