Always cache the default tools tree

Always cache the default tools tree

For the default tools tree, the cached image and the final image
are the same for all intents and purposes, so let's stop storing both
a cached image and a final image for the default tools tree and instead
only store the final image and use it as the cached image by always
writing a cache manifest next to it.

At the same time always use the name mkosi.tools to reduce the chance
of conflicts as "tools" is an incredibly common directory name.