Complete cross-compile support

Fixes found porting java/cpp/python (#14)

* Removed toolchain dependence on env, and simplified arg parsing, always work from source_dir

* Make sure docker images are re-generated for every PR to builder

* base dirs off source_dir, not launch_dir

* Fixed config mashing

* Added output when selecting git branch

* Reduced redundant cmake args

* Removed redundant cmake_binaries option

* Ensure that download script in docker images busts the cloudfront cache

* Added util to API

* Moved libcrypto to using python for download/tarball

* Prevent GC from destroying our late loaded classes

* Fixed env in cross-compile container

* All S3 uploads have .pyz extension

* Containers must download with .pyz extension

* Scoped build_env, pre_build_env, and post_build_env to just those steps