mkosi: Make sure our systemd build always overrides the distros

mkosi: Make sure our systemd build always overrides the distros

Currently, we install the systemd install tree in the base image and
then build the initrd and final images from the base image. This means
if that any systemd package is pulled in during the initrd or final
image builds, it will override our version.

To fix this, we stop installing our build of systemd in the base image,
and store it in the output directory instead. That allows us to refer to
it using ExtraTrees= in the final and initrd image builds to install it
after all the distro packages have been installed, ensuring our version
always takes priority.