Fix NuGet package path validation in nuget-stage pipeline (#40293)

Use absolute paths for artifact download and packagesToPush so that
the NuGet validation step can verify the package location. The relative
path 'drop/nuget/...' was not recognized as being under the expected
parent directory.

Co-authored-by: Ben Hillis <benhill@ntdev.microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people

.pipelines/nuget-stage.yml

@@ -34,15 +34,15 @@ stages:
       displayName: Download nuget artifacts
       inputs:
         artifact: "drop_wsl_package"
-        path: drop
+        path: $(Build.SourcesDirectory)\drop
 
       # Note: this task might fail if there's been no commits between two nightly pipelines, which is fine.
     - ${{ each package in parameters.nugetPackages }}:
         - task: NuGetCommand@2
           displayName: Push nuget/${{ package }}.$(WSL_NUGET_PACKAGE_VERSION).nupkg
           inputs:
             command: 'push'
-            packagesToPush: drop/nuget/${{ package }}.$(WSL_NUGET_PACKAGE_VERSION).nupkg
+            packagesToPush: $(Build.SourcesDirectory)\drop\nuget\${{ package }}.$(WSL_NUGET_PACKAGE_VERSION).nupkg
             nuGetFeedType: 'internal'
             publishVstsFeed: wsl
             allowPackageConflicts: ${{ parameters.isNightly }}