[nix] Add sops

Summary:
Add Mozilla SOPS to Nix shell. SOPS is a tool to manage secrets in a git repository. It's gonna be used to store encrypted Terraform secrets in the repo.
We can use it for other non-terraform purposes as well.

Test Plan: The sops command is available after nix develop / direnv reload inside the Comm repo.

Reviewers: O1 Nix, jon

Reviewed By: O1 Nix, jon

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8666