mkosi: Copy packages to the build directory as well

mkosi: Copy packages to the build directory as well

With the latest mkosi, mkosi -t none can be used to rerun the build
script without messing with a previously built image. This allows
one to run "mkosi -t disk -f qemu" in one terminal to build and boot
an image in qemu and then run "mkosi -t none" in another terminal to
rebuild the packages. If one then has "RuntimeBuildSources=yes" set
in their mkosi configuration, the build directory is mounted into the
virtual machine, which means that one can then run "dnf upgrade
/work/build/*.rpm" from within the VM to install the new packages.

This allows for quickly iterating on changes without having to rebuild
the image all the time.

We'll probably want to document this at some point, but let's start
with making it possible by copying the built packages to the build directory.