Save flatc input files when serialization fails

Summary:
It's been tough to debug `flatc` serialization problems because the files are deleted after the call.

Now, if the serialization fails, copy the input files to a directory where we can look at them, if the environment var `ET_EXIR_SAVE_FLATC_INPUTS_ON_FAILURE=1` is set.

Reviewed By: lucylq

Differential Revision: D53736362

fbshipit-source-id: 4ecd57f13ec46ba6ee0d31528f59300bc3947fd0