mkosi: Switch back to PKG_SUBDIR instead of symlinks

mkosi: Switch back to PKG_SUBDIR instead of symlinks

Previously I thought it would make sense to allow running the build
scripts from within the VM/container to rebuild the packages. Instead
we ended up making it possible to rerun mkosi outside of the container/VM
to rebuild the packages, so let's switch back to $PKG_SUBDIR to tell the
build scripts where to look for the packaging sources.

(cherry picked from commit 762b23e86a8bd45879335af96006573a99884a71)