The iptables frontend for nftables seems to not play nice. Since we use
nftables in postmarketOS anyway, tell tailscale to use it directly. This
fixes routing not working and the following error seen in tailscale status

- adding [-i tailscale0 -j MARK --set-mark 0x40000/0xff0000] in v4/filter/ts-forward: running [/usr/sbin/iptables -t filter -A ts-forward -i tailscale0 -j MARK --set-mark 0x40000/0xff0000 --wait]: exit status 2: Warning: Extension MARK revision 0 not supported, missing kernel module?

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>
Part-of: !6344