Use the NuGetPackOutput item for package validation

This is much simpler than manually combining PackageOutputPath + PackageId + PackageVersion

Thanks to Nikolche Kolev for mentioning it in NuGet/Home#6671 (comment)

Author: 0xced

src/Serilog.Formatting.Log4Net.csproj

@@ -65,7 +65,7 @@
   <Target Name="ValidateNuGetPackage" AfterTargets="Pack">
     <!-- For reference: the file `.config/dotnet-tools.json` was created by running `dotnet new tool-manifest && dotnet tool install dotnet-validate` -->
     <Exec Command="dotnet tool restore" />
-    <Exec Command="dotnet validate package local $([MSBuild]::EnsureTrailingSlash($(PackageOutputPath)))$(PackageId).$(PackageVersion).nupkg" />
+    <Exec Command="dotnet validate package local %(NuGetPackOutput.FullPath)" Condition="'%(Extension)' == '.nupkg'" />
   </Target>
 
 </Project>