Make flatc-build optional in setup.py (#3844)

Summary:
Make flatc-build optional to be able to install in editable mode.

Install ExecuTorch in editable mode with:
`EXECUTORCH_BUILD_KERNELS_CUSTOM_AOT=OFF EXECUTORCH_BUILD_FLATC=OFF pip install --editable . --config-settings editable-mode=strict --no-build-isolation`

Change-Id: I4a198d1d2372be81f55c9a9678e075d9f0640bfe

Pull Request resolved: https://github.com/pytorch/executorch/pull/3844

Reviewed By: mergennachin

Differential Revision: D58824694

Pulled By: dbort

fbshipit-source-id: 823ff0b4278d97a3619176b22e6c6085ed277e90