This is an attempt to reduce cognitive load when trying to understand the host related logic in run_machine_update.
The change should not affect behavior.
Done:
- make it very clear early on, that build_host == target_host if build_host is not set.
- rename some variables to make relations more clear
- remove `deploy_host` variable. unnecessary complexity
- remove `become_root` variable. After simplifying via boolean algebra, this boils down to `build_host == target_host`.
85dda9e125