Update mkosi.ubuntu to 'focal'

[zjs: Looking at https://packages.ubuntu.com/bionic/iptables-dev, iptables-dev
was a transitional package that was pulling in libxtables-dev, libip4tc-dev,
and libip6tc-dev (as listed by @GiedriusS). iptables-dev is gone in focal, so
replace it by the expanded list.]