Add GCE instructions to git cl creds check

R=tandrii@chromium.org

Bug: 740292
Change-Id: I0bad42e7eb58b6471ffc4bd3305677051b25e8fc
Reviewed-on: https://chromium-review.googlesource.com/660415Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>

git_cl.py

@@ -4007,7 +4007,9 @@ def CMDcreds_check(parser, args):
   _, _ = parser.parse_args(args)
 
   if gerrit_util.GceAuthenticator.is_gce():
-    DieWithError('this command is not designed for GCE, are you on a bot?')
+    DieWithError(
+        'This command is not designed for GCE, are you on a bot?\n'
+        'If you need to run this, export SKIP_GCE_AUTH_FOR_GIT=1 in your env.')
 
   checker = _GitCookiesChecker()
   checker.ensure_configured_gitcookies()