feat(fish): rebuild the flake for the running host

`nixos-rebuild switch` alone leaves the flake implicit, which is not how
this system is built. Name it explicitly, taking the host from the
configuration that installs the shell so the two cannot disagree.

The reference stays relative, so `update` rebuilds the working tree the
shell is standing in rather than a fixed checkout.

modules/fish/fish.nix

@@ -79,7 +79,7 @@ in
           jctl = "journalctl -p 3 -xb";
 
           # Rebuild the system, and reclaim disk from old generations.
-          update = "sudo nixos-rebuild switch";
+          update = "sudo nixos-rebuild switch --flake .#${config.networking.hostName}";
           cleanup = "sudo nix-collect-garbage -d";
         };