message: Update libpng to use github download instead
  One recent CI error was caused by download in
  Believe GitHub is more reliable than sourceforge. so this PR switch to
  github download instead (and mirror which is actually available in tensorflow.org)
  to reduce the chance of CI failure.
diff: @@ -315,11 +315,11 @@ http_archive(
  http_archive(
  name = "libpng",
  build_file = "//third_party:libpng.BUILD",
  - sha256 = "daeb2620d829575513e35fecc83f0d3791a620b9b93d800b763542ece9390fb4",
  + sha256 = "ca74a0dace179a8422187671aee97dd3892b53e168627145271cad5b5ac81307",
  strip_prefix = "libpng-1.6.37",
  urls = [
  - "https://storage.googleapis.com/mirror.tensorflow.org/download.sourceforge.net/libpng/libpng-1.6.37.tar.gz",
  - "https://download.sourceforge.net/libpng/libpng-1.6.37.tar.gz",
  + "https://storage.googleapis.com/mirror.tensorflow.org/github.com/glennrp/libpng/archive/v1.6.37.tar.gz",
  + "https://github.com/glennrp/libpng/archive/v1.6.37.tar.gz",
  ],
  )
  