Store update-translations tempfiles in build workspace

Summary:
download_i18n.py creates temporary directories that are currently about
~1.4GB in size. On the Jenkins machine this quickly fills up the root
volume. Since these files may be useful for debugging and there is
plenty of space on the EBS volume we will save them into the build
workspace and keep fewer builds archives around (20) to cap the amount
of data stored.

Test Plan: Fingers crossed.

Reviewers: csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D3667