nixos/dnscrypt-proxy2: service restart on failure

it does happen that `dnscrypt-proxy` exit when it is unable to
synchronise its resolvers metadata on startup. this can happen due
to network connectivity issues for example. not restarting it automatically
means no dns resolution will work until a manual restart is performed.

nixos/modules/services/networking/dnscrypt-proxy2.nix

@@ -55,6 +55,7 @@ in
         AmbientCapabilities = "CAP_NET_BIND_SERVICE";
         DynamicUser = true;
         ExecStart = "${pkgs.dnscrypt-proxy2}/bin/dnscrypt-proxy -config ${cfg.configFile}";
+        Restart = "always";
       };
     };
   };