wgengine/router: use iptablesRunner when no firewall tool is available:

The current router errors out when neither iptables nor nftables support is present. We
should fall back to the previous behaviour which we creates a dummy iptablesRunner.

Fixes: #8878
Signed-off-by: KevinLiang10 <kevinliang@tailscale.com>

wgengine/router/router_linux_test.go

@@ -1116,7 +1116,7 @@ func TestChooseFireWallMode(t *testing.T) {
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			got, _ := chooseFireWallMode(t.Logf, tt.det)
+			got := chooseFireWallMode(t.Logf, tt.det)
 			if got != tt.want {
 				t.Errorf("chooseFireWallMode() = %v, want %v", got, tt.want)
 			}