refactor: don't include the whole modules

Instead only import the nixos/hm/nixvim module.

This _should_ reduce rebuilds.

example-vm/default.nix

@@ -1,5 +1,5 @@
 {
-  import = [ ./vm_config.nix ];
+  imports = [ ./vm_config.nix ];
 
   time.timeZone = "Europe/Berlin";
   i18n.defaultLocale = "en_US.UTF-8";