move root's home directory to /home/root

/ is a bad choice of home directory

/home/root instead of /root is to make it easier to write
ssh keys to the right place

modules/base.nix

@@ -107,7 +107,7 @@ in {
     ];
     users.root = {
       uid = 0; gid= 0; gecos = "Root of all evaluation";
-      dir = "/";
+      dir = "/home/root/";
       passwd = lib.mkDefault "";
       shell = "/bin/sh";
     };