\#20629 moved the mkosi configs to mkosi.default.d/ so we were building
for the host distro (Ubuntu) in each CI configuration. To fix it, we
write the distro we want to test to a mkosi.default file and mkosi
will apply the other necessary configs automatically from mkosi.default.d/<distro>
This commit also removes unnecessary CLI options that are already handled
by the config files.
Daan De Meyer
7e99216c1d