mkosi: Use build image prepare scripts for tools tree as well (#36861)

Instead of listing dependencies manually for the default tools tree,
let's reuse the prepare scripts from the build image. To make this work,
the sync script has to be configured for the tools tree as well so that
it's invoked both when building the tools tree and for the regular
image,
otherwise, when doing the first build in a fresh checkout, the sync
script
won't have executed yet as sync scripts for the regular images are
executed
after building the default tools tree.