Now that we have the mkosi.clangd script to run clangd from the mkosi
build script, it becomes clear that doing cleanup with mkosi.clean has
a big gap in that we always run the mkosi.clean script and thus we also
run it when we run the mkosi.clangd script, causing the previously built
packages to be removed when we run clangd without producing new ones.
In mkosi we're improving the situation by only running clean scripts when we
clean up the output directory and disallowing writing to the output directory
from build scripts.
Let's adapt systemd to these changes by moving the copying of packages to the
output directory to the postinst script.
