Update env to workaround MI (Azure#37675)

Author: xiangyan99

sdk/appconfiguration/tests.yml

@@ -5,7 +5,7 @@ parameters:
     type: object
     default:
       - azure-appconfiguration
-      # - azure-appconfiguration-provider
+      - azure-appconfiguration-provider
 
 extends:
     template: ../../eng/pipelines/templates/stages/archetype-sdk-tests.yml
@@ -35,3 +35,6 @@ extends:
         TEST_MODE: 'RunLiveNoRecord'
         AZURE_SKIP_LIVE_RECORDING: 'true'
         AZURE_TEST_RUN_LIVE: 'true'
+        # Set fake authority host to ensure Managed Identity fail for Default Azure Credential
+        # so "execute samples" step correctly picks up Powershell credential.
+        AZURE_POD_IDENTITY_AUTHORITY_HOST: "FakeAuthorityHost"