Ban the silent version

tools/run_tests/sanity/core_banned_functions.py

@@ -41,6 +41,8 @@ BANNED_EXCEPT = {
     'grpc_closure_sched(' : ['src/core/lib/iomgr/closure.c'],
     'grpc_closure_run(' : ['src/core/lib/iomgr/closure.c'],
     'grpc_closure_list_sched(' : ['src/core/lib/iomgr/closure.c'],
+    'gpr_getenv_silent(' : ['src/core/lib/support/log.c', 'src/core/lib/support/env_linux.c', 
+                            'src/core/lib/support/env_posix.c', 'src/core/lib/support/env_windows.c'],
 }
 
 errors = 0