Rework version bumping

Rework version bumping

Currently, --auto-bump= bumps the version after finishing a build.
Effectively this means the version in mkosi.version doesn't reflect
the actual version of the image which seems weird. Instead, let's
generate the new version upfront so that it's used by the build but
only write it to mkosi.version when we do a successful build.

Additionally, look for a mkosi.bump script and use it if it exists
to generate the new version so we can generate a timestamp as the
version in particleos.