refactor: remove type restriction on machine option

There's no real need to be so restrictive about what kinds of machine
names there can be.

machine.nix

@@ -2,6 +2,6 @@
 {
   options.machine = lib.mkOption {
     description = "The machine hostname to configure for";
-    type = lib.types.enum [ "earth" "fire" "air" "water" ];
+    type = lib.types.str;
   };
 }