Broken DHCPv4 in incus-vm

If anyone else gets caught up on this, the problem is that systemd checks UDP checksums:

# SYSTEMD_LOG_LEVEL=debug /lib/systemd/systemd-networkd
...
enp5s0: DHCPv4 client: DISCOVER
ignoring packet: invalid UDP checksum

Forwarded the bug to incus, which is in a position to do a workaround: https://github.com/lxc/incus/issues/1997

The good news is that debusine.debian.net has IPv6-only workers, so missing IPv4 shouldn't be a blocker to anything.

The workaround is:

nft insert rule inet incus out.debusinebr0 index 0 oifname "debusinebr0" ip protocol udp udp checksum set 0;

Or similar built into the host's firewall