nixos/postfix-tlspol: only preset dns resolver with useLocalResolver

This is the best indicator we have about whether to use a local resolver.

In the meantime I'm lobbying upstream to read /etc/resolv.conf.

(cherry picked from commit b201963951)

nixos/modules/services/mail/postfix-tlspol.nix

@@ -8,6 +8,7 @@
 let
   inherit (lib)
     hasPrefix
+    literalExpression
     mkEnableOption
     mkIf
     mkMerge
@@ -92,7 +93,13 @@ in
           dns = {
             address = mkOption {
               type = types.str;
-              default = "127.0.0.1:53";
+              default = if config.networking.resolvconf.useLocalResolver then "127.0.0.1:53" else null;
+              defaultText = literalExpression ''
+                if config.networking.resolvconf.useLocalResolver then
+                  "127.0.0.1:53"
+                else
+                  null
+              '';
               description = ''
                 IP and port to your DNS resolver