toolchain/build.sh: clean up post-build artifacts

We need to clean out some temp files in order to follow up with
packaging. We don't want the build artifacts to be included in the
source tarball.
toolchain/build.sh: set ZRT_ROOT env var for pkg

The packaging stage expects the ZRT_ROOT var to be set, so this changes
makes sure that env var configuration is set in the ~/.bashrc of the LXC
(so that it guaranteed to be set when we execute remote commands).