abuild-rootbld: clear environment for bwrap container

Otherwise, user-set environment variables can leak into the container and cause spurious build/test failures. A common example is the value of the SHELL environment variable which is used by a lot of software. Outside of the bwrap container I use ksh and my SHELL environment variable points to /bin/ksh, however, inside the container /bin/ksh is not available and hence software relying on $SHELL doesn't work properly. This can cause annoying to debug test failures, e.g. aports!43430 (merged).