Environment variable unsetting sometimes fails
Created by: ollieparanoid
It 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, we should instrument env
to create an empty environment (like it is done in Alpine's Travis CI scripts).