Update usage of os.IsNotExist (#29)

Per [godoc](https://pkg.go.dev/os#IsNotExist), this is the preferred method.

pkg/update/filestore/filestore.go

@@ -54,9 +54,9 @@ func (s *fileStore) GetMeta() (map[string]json.RawMessage, error) {
 
 func (s *fileStore) readData() error {
 	stat, err := os.Stat(s.filename)
-	if err != nil && !os.IsNotExist(err) {
+	if err != nil && !errors.Is(err, os.ErrNotExist) {
 		return errors.Wrap(err, "stat file store")
-	} else if os.IsNotExist(err) {
+	} else if errors.Is(err, os.ErrNotExist) {
 		// initialize empty
 		s.metadata = metadataMap{}
 		return nil

pkg/update/update.go

@@ -235,7 +235,7 @@ func (u *Updater) Download(repoPath, localPath string) error {
 
 	if constant.PlatformName == "windows" {
 		// Remove old file first
-		if err := os.Rename(localPath, localPath+".old"); err != nil && !os.IsNotExist(err) {
+		if err := os.Rename(localPath, localPath+".old"); err != nil && !errors.Is(err, os.ErrNotExist) {
 			return errors.Wrap(err, "rename old")
 		}
 	}