nixos/tests/k3s: fix multi-node test

This corrects the multi-node test after a couple recent changes which
resulted in it being broken.

The `lib.toString` change was an incorrect tree-wide refactor, and the
aarch64 change also introduced an error in python indentation/formatting
I believe.

nixos/tests/k3s/multi-node.nix

@@ -112,7 +112,7 @@ import ../make-test-python.nix ({ pkgs, lib, ... }:
           enable = true;
           role = "agent";
           serverAddr = "https://192.168.1.3:6443";
-          extraFlags = lib.toString [
+          extraFlags = lib.concatStringsSep " " [
             "--pause-image" "test.local/pause:local"
             "--node-ip" "192.168.1.2"
           ];
@@ -138,12 +138,15 @@ import ../make-test-python.nix ({ pkgs, lib, ... }:
           m.start()
           m.wait_for_unit("k3s")
 
+      is_aarch64 = "${toString pkgs.stdenv.isAarch64}" == "1"
+
       # wait for the agent to show up
       server.wait_until_succeeds("k3s kubectl get node agent")
 
       for m in machines:
-      '' # Fix-Me: Tests fail for 'aarch64-linux' as: "CONFIG_CGROUP_FREEZER: missing (fail)"
-      + lib.optionalString (!pkgs.stdenv.isAarch64) ''m.succeed("k3s check-config")'' + ''
+          # Fix-Me: Tests fail for 'aarch64-linux' as: "CONFIG_CGROUP_FREEZER: missing (fail)"
+          if not is_aarch64:
+              m.succeed("k3s check-config")
           m.succeed(
               "${pauseImage} | k3s ctr image import -"
           )