ci(spanner): change directpath environment to cloud-staging

Author: sakthivelmanii

.kokoro/build.sh

@@ -113,6 +113,7 @@ integration-directpath-enabled)
       -Dclirr.skip=true \
       -Denforcer.skip=true \
       -Dmaven.main.skip=true \
+      -Dspanner.gce.config.server_url=https://preprod-spanner.sandbox.googleapis.com \
       -Dspanner.testenv.instance=projects/span-cloud-testing/instances/spanner-java-client-directpath \
       -Dspanner.gce.config.project_id=span-cloud-testing \
       -fae \