feat: switch to fish as an interactive shell

Now that it is written in Rust I am interested!

flake-modules/example-vm.nix

@@ -12,7 +12,7 @@ in
     modules = [
       inputs.self.nixosModules.default
       ../example-vm # import vm configuration
-      { nix.registry.nixpkgs.flake = inputs.nixpkgs; }
+      { nix.registry.nixpkgs.flake = inputs.nixpkgs; } # pin nixpkgs to the one used by the system
     ];
   };