Fix unintended breaking change

- blocking dotnet/aspnetcore#1293 dependency update PR
- commit 3ec8c35 lost `AssemblyName` substitution
  - tasks assembly in package incorrectly named ".Manifest.Task.dll"

Author: dougbu

src/FileProviders/Embedded/src/Microsoft.Extensions.FileProviders.Embedded.csproj

@@ -26,7 +26,7 @@
   </ItemGroup>
 
   <ItemGroup>
-
+    <NuspecProperty Include="AssemblyName=$(AssemblyName)" />
     <NuspecProperty Include="OutputBinary=$(OutputPath)**\$(AssemblyName).dll" />
     <NuspecProperty Include="OutputSymbol=$(OutputPath)**\$(AssemblyName).pdb" />
     <NuspecProperty Include="OutputDocumentation=$(OutputPath)**\$(AssemblyName).xml" />