eng, fix preview pipeline after microsoft/typespec refactor (#2960)

Author: weidongxu-microsoft

eng/pipelines/preview.yml

@@ -39,3 +39,7 @@ extends:
     - name: emitter
       file: azure-tools-typespec-java-*.tgz
       type: npm
+    AutorestInitializationSteps:
+    - checkout: self
+      submodules: true
+      path: s/autorest

eng/scripts/Build-Packages.ps1

@@ -26,7 +26,7 @@ $packagesPath = New-Item -ItemType Directory -Force -Path $packagesPath | Select
 
 Push-Location $RepoRoot
 try {
-  invoke "mvn -f pom.xml -P local,tsp -T 1C --no-transfer-progress clean install"
+  invoke "pwsh Build-TypeSpec.ps1"
 
   Push-Location "./typespec-extension"
   try {