jiri: Use runtil.IsNotExist instead of os.IsNotExist.

The Sequence helper wraps its errors, so we must use runtil.IsNotExist
to see if the new error is a NotExist error, instead of relying on
on.IsNotExist, which only works for unwrapped errors.

Change-Id: I11941f6307935a5325f9ead8cb98705bfb704ce1
diff --git a/gerrit/gerrit.go b/gerrit/gerrit.go
index 0d7044b..08f96f2 100644
--- a/gerrit/gerrit.go
+++ b/gerrit/gerrit.go
@@ -462,7 +462,7 @@
 	netrcPath := filepath.Join(os.Getenv("HOME"), ".netrc")
 	file, err := seq.Open(netrcPath)
 	if err != nil {
-		if !os.IsNotExist(err) {
+		if !runutil.IsNotExist(err) {
 			return nil, err
 		}
 	} else {
@@ -484,7 +484,7 @@
 		cookieFilePath := strings.TrimSpace(stdout.String())
 		file, err := seq.Open(cookieFilePath)
 		if err != nil {
-			if !os.IsNotExist(err) {
+			if !runutil.IsNotExist(err) {
 				return nil, err
 			}
 		} else {