Fix path to dotnet validate package local command

Author: 0xced

src/Serilog.Formatting.Log4Net.csproj

@@ -55,7 +55,7 @@
 
   <Target Name="ValidateNuGetPackage" AfterTargets="Pack">
     <Exec Command="dotnet tool restore" />
-    <Exec Command="dotnet validate package local @(NuGetPackOutput)" Condition="%(NuGetPackOutput.extension) == '.nupkg'" />
+    <Exec Command="dotnet validate package local $([MSBuild]::EnsureTrailingSlash($(PackageOutputPath)))$(PackageId).$(PackageVersion).nupkg" />
   </Target>
 
   <Target Name="PrintPackageVersionForGitHubActions" AfterTargets="Pack">