Drop the work host

Drop the work host The work machine does not exist, so nix/hosts/work.nix declared a hostname this repo invented and nothing else: every cask, App Store app, package and macOS default came from the shared modules, making the work host a byte-identical build of personal with a different computer name. The divergence its own comment promised was never written. It was also a small footgun, since `make switch HOST=work` on this machine would have renamed it for no benefit. With one host the surrounding indirection has nothing left to hold, so the mkHost factory, the specialArgs hostname, and nix/hosts/ all go too. The three networking options move into system.nix and the Makefile and bootstrap.sh stop taking a host argument. The argument for scaffolding two hosts early was avoiding a refactor of the shared module layout later. That layout now exists regardless, so reintroducing a second host is a small, local change if a work Mac ever turns up.