Introduce --run-build-scripts (-R) to always run build scripts

Introduce --run-build-scripts (-R) to always run build scripts

Currently we run "mkosi -t none" in systemd and mkosi-kernel to run
the build script(s) without rebuilding the entire image. Using the
"none" output format for this purpose is a hack. It also breaks when
using History=yes as running mkosi -t none will change the output
format in the history after which running mkosi qemu will fail saying
it can't boot the "none" output format.

Let's introduce a --run-build-scripts argument which will make us always
run the build scripts regardless of whether the image has already been
built or not. At the same time, remove various behaviors from the "none"
format that were solely added to enable the hack of using it to run build
scripts without rebuilding the image:

- Clean up outputs from the previous build when the "none" output format
  is used