Zap in 'pmbootstrap init' removes chroots in *previous* work folder

Created by: ollieparanoid

@BrianOtto wrote in https://github.com/postmarketOS/pmbootstrap/issues/501#issuecomment-327408580:

Any time I ran ./pmbootstrap.py init the chroot_native directory (e.g. /home/pmos/.local/var/pmbootstrap-test4/chroot_native) would get removed in my PREVIOUS test directory. For example, when I was working in pmbootstrap-test5, the pmbootstrap-test4 one got removed. I thought that by creating a new directory and setting the Work Path to this, the changes would be isolated. So either I am misunderstanding how the process works, or the script is accessing previous build files when it shouldn't.