Allow hostsettings.json and normal DOTNET_ environment variables?

https://github.com/dotnet/maui/blob/4340ccf4f87ffae1bf146c5ffe4908e53bd72879/src/Core/src/Hosting/AppHostBuilder.cs#L139


[Documentation listed here](https://docs.microsoft.com/en-us/aspnet/core/fundamentals/host/generic-host?view=aspnetcore-5.0#ihostenvironment) about the environment variables to use for GenericHost.