Call 'mkosi.finalize' script

Call 'mkosi.finalize' script

This is similar to 'mkosi.postinst', but is called in the host. This
makes it much easier to copy-in files. It is also much quicker when
creating images for a foreign architecture.

v2:
- s/postinst2/finalize/g
- call the script first with "build" too
- pass the root directory as $BUILDROOT (like rpmbuild) and not as
  a positional argument. (This is easier to consume for scripts and
  also easier to extend with additional variables in the future.)
- update to use args_find_path()