mkosi CI: mask isc-dhcp-server units

mkosi CI: mask isc-dhcp-server units

The packages are installed to provide the dhcpd binary, used by
test/test-network/systemd-networkd-tests.py, but we don't need the units
to run, and in fact in some cases the image fails to boot because of
them:

Spawning container image on /home/runner/work/systemd/systemd/image.raw.
Press ^] three times within 1s to kill container.
● isc-dhcp-server.service  loaded failed failed ISC DHCP IPv4 server
● isc-dhcp-server6.service loaded failed failed ISC DHCP IPv6 server
Container image failed with error code 1.
Error: Process completed with exit code 1.

Mask the units with an --extra-tree.