Add an option to remove the working directory after a successful build

Running sudo rm -rf /tmp/path/to/archiso/work_dir/ after each successful build can be annoying.

Let's provide a command line flag to remove the working directory, but only if we created it in the first place.

Keeping the working directory intact should still remain the default as it can be useful when testing things (e.g. boot mode related stuff that doesn't need changes to the airootfs).