Add NuGet.Build.Tasks.Pack package

This should re-enable per-PR releases to MyGet

Author: lukebakken

projects/client/RabbitMQ.Client/RabbitMQ.Client.csproj

@@ -51,6 +51,10 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0" PrivateAssets="All" />
+    <PackageReference Include="NuGet.Build.Tasks.Pack" Version="5.4.0">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
   </ItemGroup>
 
   <ItemGroup Condition=" '$(Configuration)' == 'SignedRelease' ">