testgrid: Upstream our tahoe-lafs NixOS module

Our NixOS module for Tahoe-LAFS already now is better (actually works, more secure, adheres more closely to NixOS/systemd conventions) than [what's in nixpkgs](https://github.com/NixOS/nixpkgs/blob/nixos-24.11/nixos/modules/services/network-filesystems/tahoe.nix).
Let's upstream it when it's done (looking at #14, #8, #7 especially).