feat(systems): add tortoise NixOS host configuration

Registers the tortoise machine in flake.nix and adds its system
configuration with a placeholder hardware config (ZFS root pool,
x86_64) to be replaced by the installer-generated hardware-configuration.nix.

users/richie/systems/tortoise.nix

@@ -0,0 +1,5 @@
+{
+  imports = [
+    ../home/global.nix
+  ];
+}