Fix #300: Properly unset environment in chroot

Created by: ollieparanoid

Unsetting the environment variables fails in Travis CI sometimes:

• https://travis-ci.org/postmarketOS/binary-package-repo/builds/259167394#L359
• https://travis-ci.org/postmarketOS/pmbootstrap/builds/259175393#L619

Instead of unsetting all environment variables, this commit instruments env to create an empty environment (like it is done in Alpine's Travis CI scripts).