Instead of (optionally) building the image when this option is enabled,
let's insist that the image has already been built and cached. This allows
us to reuse the history of the previous build if History= is enabled. At
the same time, rename the option to --rerun-build-scripts to indicate more
clearly that the image needs to have been built once already.
This allows the option to be used in systemd's mkosi.clangd script to replace
"-t none" and "--incremental=strict" there.