Remove network checksum offloading on build hosts

Based on a hint @ https://github.com/virtio-win/kvm-guest-drivers-windows/discussions/1371 we should set the rx checksum offload to off. Based on tests it definitely is so. Depending on OS and transfer speeds, the improvement is  1.3x - 4x.

This should be done on the host interfaces, not bond0 or br0, but the interfaces eno12399np0 and eno12409np1.

> ethtool -K eno12399np0 rx off
> ethtool -K eno12409np1 rx off

This will however be reset during boot, so let's create this:
/etc/systemd/system/disable-offloads.service

[Unit]
Description=Disable RX Offloads
After=network-online.target
Wants=network-online.target

[Service]
Type=oneshot
ExecStart=/sbin/ethtool -K enp5s0f0 rx off
ExecStart=/sbin/ethtool -K enp5s0f1 rx off
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

Obviously you have to check the name of the interface here and not just blindly copy paste.
Then
> systemctl daemon-reload
> systemctl enable disable-offloads.service
> systemctl start disable-offloads.service (if you want to run it right away and disable the offload (if you didn't run the ethtool above))