core/iwasm/libraries/wasi-nn/test/build.sh: add a tip for intel mac (#4389)

i keep forgetting this and had to re-investigate it at least twice.
hopefully this can be helpful for others too.

core/iwasm/libraries/wasi-nn/test/build.sh

@@ -3,6 +3,17 @@
 # Copyright (C) 2019 Intel Corporation.  All rights reserved.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
+# on intel mac, this ends up with a lot of the following error.
+#
+#  AttributeError: 'Sequential' object has no attribute '_get_save_spec'.
+#
+# * "pip install tensorflow" installs tensorflow 2.16.2 on intel mac.
+#   (because it's the last version before tf deprecated the target.)
+# * keras 3 support in the version seems incomplete (thus the error)
+# * a workaround: use keras 2 as mentioned in:
+#   https://github.com/tensorflow/tensorflow/releases/tag/v2.16.1
+#   https://blog.tensorflow.org/2024/03/whats-new-in-tensorflow-216.html
+
 CURR_PATH=$(cd $(dirname $0) && pwd -P)
 
 # WASM application that uses WASI-NN