add install node step

Author: chunyu3

packages/typespec-vscode/eng/pipeline/templates/ci-stages.yml

@@ -17,4 +17,11 @@ stages:
       - job: UI_Test
         displayName: "UI Test"
         steps:
+          - task: NodeTool@0
+            displayName: Install Node.js
+            retryCountOnTaskFailure: 3
+            inputs:
+              versionSpec: 22.x
+          - script: npm install -g pnpm
+            displayName: Install pnpm
           - template: /eng/tsp-core/pipelines/templates/build.yml