add windows portability build_only job

tools/internal_ci/windows/grpc_portability_master.cfg → tools/internal_ci/windows/grpc_portability_build_only.cfg

@@ -26,5 +26,5 @@ action {
 
 env_vars {
   key: "RUN_TESTS_FLAGS"
-  value: "-f portability windows -j 1 --inner_jobs 8 --internal_ci"
+  value: "-f portability windows --internal_ci  --build_only"
 }