message: nydusify: remove single blob file after `upload` phase
  After blob file being uploaded to either oss or registry, it is
  nolonger needed. Removing them will save local storage backend.
diff: @@ -114,6 +114,8 @@ func (layer *buildLayer) pushBlob(ctx context.Context) error {
  return errors.Wrap(err, "Stat blob file")
  }
  + defer os.Remove(blobPath)
  +
  if err := utils.WithRetry(func() error {
  size := info.Size()
  desc, err := layer.backend.Upload(ctx, blobID, blobPath, size)
  @@ -298,6 +300,8 @@ func (layer *buildLayer) Build(ctx context.Context) error {
  if err != nil {
  return buildDone(errors.Wrapf(err, "Build source layer %s", layer.source.Digest()))
  }
  +
  + // The built blob will be removed after `upload` phrase.
  layer.blobPath = blobPath
  return buildDone(nil)
  