mkarchiso can't retry where it left off if it fails after _build_iso_base

To test, add exit 1 at the end of _build_iso_image and try running mkarchiso -m 'iso' twice.

It results in:

./archiso/mkarchiso: line 789: isofs_dir: unbound variable

Or with exit 1 at the end of _export_netboot_artifacts and mkarchiso -m 'netboot':

./archiso/mkarchiso: line 672: isofs_dir: unbound variable

The problem is that _build_iso_base sets essential variables such as $isofs_dir, so it should not be run with _run_once.