reclient: show warning if $RBE_instance is set

Bug: b/287174095
Change-Id: Ia8c89e36490e4a204582c566ad654fcba9aa7aaa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4616940
Commit-Queue: Fumitoshi Ukai <ukai@google.com>
Commit-Queue: Richard Wang <richardwa@google.com>
Auto-Submit: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Richard Wang <richardwa@google.com>

reclient_helper.py

@@ -190,6 +190,10 @@ def build_context(argv, tool):
   if reclient_metrics.check_status(ninja_out):
     set_reproxy_metrics_flags(tool)
 
+  if os.environ.get('RBE_instance', None):
+    print('WARNING: Using RBE_instance=%s\n' %
+          os.environ.get('RBE_instance', ''))
+
   reproxy_ret_code = start_reproxy(reclient_cfg, reclient_bin_dir)
   if reproxy_ret_code != 0:
     yield reproxy_ret_code