Fix install_flatc.sh version checking (#2402)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2402

Instead of looking at the latest git tag, look at the contents of a file in the repo.

The old logic broke when upstream released a new version, but we were still pinned to the old version.

This was functionally harmless, but printed a misleading message to users.

Reviewed By: shoumikhin

Differential Revision: D54863485

fbshipit-source-id: eaaf7ebc148224298f06ce7f8418efb159692f44