Remove BASH_ENV from the build environment

This runs an arbitrary shell script for all non-interactive bash sessions, and we've got lots of those.

It was causing problems with Fedora 30, which triggers the `manpath` executable, which isn't available in our $PATH:
https://groups.google.com/d/msg/android-building/WqEyZQuxYtY/zDXl9irOCAAJ

Change-Id: If2f63bbbcc3accb8d239f93fa06823d301d69488