Add build extensions to GalleryTools (#8399)

Author: joelverhagen

src/GalleryTools/GalleryTools.csproj

@@ -107,4 +107,5 @@
   </PropertyGroup>
   <Import Project="$(SignPath)\sign.targets" Condition="Exists('$(SignPath)\sign.targets')" />
   <Import Project="$(SignPath)\sign.microbuild.targets" Condition="Exists('$(SignPath)\sign.microbuild.targets')" />
+  <Import Project="$(NuGetBuildExtensions)" Condition="'$(NuGetBuildExtensions)' != '' And Exists('$(NuGetBuildExtensions)')" />
 </Project>