Update powershell script

.ci/azure-build.yml

@@ -31,7 +31,9 @@ steps:
 - bash: ls ${ARTIFACT_STAGING_DIRECTORY}
   env:
     ARTIFACT_STAGING_DIRECTORY: $(Build.ArtifactStagingDirectory))
-- pwsh: if( (Get-ChildItem $(Build.ArtifactStagingDirectory) | Measure-Object).Count -eq 0) { echo '##vso[task.setvariable variable=buildSuccess]false' } else { echo '##vso[task.setvariable variable=buildSuccess]true' }
+- pwsh: if( (Get-ChildItem $ENV:ARTIFACT_STAGING_DIRECTORY | Measure-Object).Count -eq 0) { echo '##vso[task.setvariable variable=buildSuccess]false' } else { echo '##vso[task.setvariable variable=buildSuccess]true' }
+  env:
+    ARTIFACT_STAGING_DIRECTORY: $(Build.ArtifactStagingDirectory))
 # delete the release if it already exists to avoid an error
 - task: GitHubRelease@0
   condition: and(succeeded(), eq(variables['buildSuccess'], 'true') )