Fix meta.tld option

The option curretly keeps it default value, even if set. This fixes the
value being passed through correctly, so that the user-specified value
is actually used.

Fixes: #5669

nixosModules/clanCore/metadata.nix

@@ -107,8 +107,7 @@ in
             readOnly = true;
           };
           tld = lib.mkOption {
-            default = "clan";
-            type = lib.types.str;
+            type = types.strMatching "[a-z]+";
             description = ''
               the TLD for the clan
             '';