Ensure GenerateRuntimeJson runs before Pack

The pack task itself doesn't do anything but depend on a sequence of tasks.
As a result running a target `BeforeTargets="Pack"` actually runs after
the package is created, since dependencies run before BeforeTargets.

Sequence the target before GenerateNuspec instead.

src/libraries/Microsoft.NETCore.Platforms/src/Microsoft.NETCore.Platforms.csproj

@@ -48,7 +48,7 @@
     <PackageReference Include="NuGet.ProjectModel" Version="$(RefOnlyNugetProjectModelVersion)" />
   </ItemGroup>
 
-  <Target Name="GenerateRuntimeJson" Condition="'$(AdditionalRuntimeIdentifiers)' != ''" BeforeTargets="Pack">
+  <Target Name="GenerateRuntimeJson" Condition="'$(AdditionalRuntimeIdentifiers)' != ''" BeforeTargets="GenerateNuspec">
     <MakeDir Directories="$(IntermediateOutputPath)" />
     <GenerateRuntimeGraph RuntimeGroups="@(RuntimeGroupWithQualifiers)"
                           AdditionalRuntimeIdentifiers="$(AdditionalRuntimeIdentifiers)"