Reenable kea dhcpv6 on core machines

[sarcasticadmin]

Description

Currently kea dhcpv6 daemon is disabled:

scale-network/nix/machines/core/common.nix

Line 62 in c6e657d

enable = false;

due to errors within the dhcpv4 and dhcpv6 daemons clobbering each others pids (the logs werent originally captured when the issue was happening). I believe this was due to the version of nixpkgs were currently pinned to that happened to include a previous change that was ultimately reverted: NixOS/nixpkgs#274460

Acceptance Criteria

Get the two daemons to share a common instance of /var/run/kea

[owendelong]

The issue isn't just the PIDs.

The issue is the entire /var/run/kea directory.

Each creates its own instance of that directory (some form of kernel namespace systemd thing I admit I don't fully understand and haven't encountered prior to our use of NIX).

Whichever one starts last ends up being the only one visible to users on the system. It is possible there is some way to access the other one which I am unaware of.

Since I was writing debug log files in that directory to overcome systems logging limitations, this was problematic.

[owendelong]

Yes, the bug description for NixOS 2774460 appears to be exactly what we encountered here.

[sarcasticadmin]

Thanks for the additional context @owendelong

[owendelong]

Marking critical as we really want to make sure this is addressed before the show.