Refactor prepare-pipelines.yml (#31003)

eng/pipelines/prepare-pipelines.yml

@@ -1,10 +1,9 @@
 trigger: none
 
+variables:
+- template: /eng/pipelines/templates/variables/image.yml
+
 extends:
   template: /eng/common/pipelines/templates/jobs/prepare-pipelines.yml
   parameters:
-    Repository: $(Build.Repository.Name)
-    Prefix: js
-    CIConventionOptions: ''
-    UPConventionOptions: '--variablegroups 24 58 76 93'
-    TestsConventionOptions: '--variablegroups 64'
+    Repository: Azure/azure-sdk-for-js