[nuget] should specify $(PackageType) of MSBuildSdk (#15)

Context: https://docs.microsoft.com/nuget/create-packages/set-package-type
Context: 7bf8b6484b/Tools/MSBuild.Packaging.targets (L4)

Xamarin.Legacy.Sdk needs to specify it's NuGet package type.

src/Xamarin.Legacy.Sdk/Xamarin.Legacy.Sdk.csproj

@@ -2,6 +2,7 @@
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
     <PackageVersion>0.1.0-alpha2</PackageVersion>
+    <PackageType>MSBuildSdk</PackageType>
     <OutputPath>../../bin/$(Configuration)/</OutputPath>
     <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
     <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>