tstest/integration/vms: test on stable nixos (21.11)

I would like to do some more customized integration tests in the future,
(specifically, bringing up a mitm proxy and testing tailscaled through that)
so hoping to bring back the nixos wiring to support that.

Signed-off-by: Tom DNetto <tom@tailscale.com>

tstest/integration/vms/top_level_test.go

@@ -18,3 +18,9 @@ func TestRunUbuntu2004(t *testing.T) {
 	setupTests(t)
 	testOneDistribution(t, 1, Distros[1])
 }
+
+func TestRunNixos2111(t *testing.T) {
+	t.Parallel()
+	setupTests(t)
+	testOneDistribution(t, 2, Distros[2])
+}