Fixed targets not imported during multitargeting

The "buildMultiTargeting\NuGet.Build.Packaging.MultiTargeting.targets"
was not imported into msbuild script during multi-target phase, due to
the name not matching the package id.
Changed
"buildMultiTargeting\NuGet.Build.Packaging.MultiTargeting.targets"
to
"buildMultiTargeting\NuGet.Build.Packaging.targets",
removing the MultiTargeting part from the file name.

Related to #6277

Author: ChristophLindemann

src/Build/NuGet.Build.Packaging.Tasks/NuGet.Build.Packaging.Tasks.csproj

@@ -26,7 +26,7 @@
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </Content>
     <Content Update="NuGet.Build.Packaging.MultiTargeting.targets">
-      <PackagePath>buildMultiTargeting\%(Filename)%(Extension)</PackagePath>
+      <PackagePath>buildMultiTargeting\NuGet.Build.Packaging%(Extension)</PackagePath>
     </Content>
   </ItemGroup>
   <ItemGroup>