nixos/userborn: convert users.users.<name>.home to string

If `opts.home` is not a string, an evaluation failure occurs in
`mapAttrs'`. Converting `opts.home` to string solves this issue.

The type of `users.users.<name>.home` is `lib.types.path`. Values
other than strings, e.g., derivations can also have the type
`lib.types.path`.

nixos/modules/services/system/userborn.nix

@@ -97,7 +97,7 @@ in
       # home.
       tmpfiles.settings.home-directories = lib.mapAttrs' (
         username: opts:
-        lib.nameValuePair opts.home {
+        lib.nameValuePair (toString opts.home) {
           d = {
             mode = opts.homeMode;
             user = username;