Pin PyTorch nightly in Executorch Linux CI (20230731) (#31)

Summary:
Per our discussion today, let's pin PyTorch nightly for now till we have CI in place to safely update this.  The nightly version could be updated manually

Pull Request resolved: https://github.com/pytorch/executorch/pull/31

Reviewed By: dbort

Differential Revision: D47923775

Pulled By: huydhn

fbshipit-source-id: c2bd6e4e0db620369b4f1e7cc6ff3c867e55923c