mkosi: when using cached images, randomize fs and partition uuids explicitly

mkosi: when using cached images, randomize fs and partition uuids explicitly

This addresses an annoying issue when building cached images containing
btrfs file systems: the kernel btrfs driver refuses mounting or handling
multiple different file systems with the same uuid. This means using an
image and building the next at the same time fails — as long as the
UUIDs of the newly build image aren't refreshed.

This patches makes sure when using a cached image we'll refresh disk,
partition and file system uuids. We generate them randomly, exactly like
we would have them when using non-cached builds.

This also ensures that the partition labels are rewritten when images
versions are bumped.

(Eventually we should probably start hashing the uuids from the
configuration state in some form, to provide a certain level of
reproducibility, but for now let's just randomize them.)