[Nix] Update flake structure

Summary:
Update flake structure to reflect changes in the nix ecosystem.
nix flake check will tell you that certain attributes are deprecated.

nixConfig is a top-level flake attribute, placing it in the correct location
fixes the cache usage. comm.cachix.org now correctly serves up dependencies.

https://linear.app/comm/issue/ENG-1838
Related: (Dev shell) https://linear.app/comm/issue/ENG-1837

Test Plan: nix flake check doesn't return deprecation warnings

Reviewers: atul, abosh, varun, O1 Nix

Reviewed By: varun, O1 Nix

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5167