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