Speed up daily deploy by reusing staging workspace

Summary:
We already speed up deployments by keeping genfiles/ between builds to
avoid rebuilding files whose dependencies haven't changed.

Usually each build had its own workspace. Staging builds are fast
because the deltas between builds are small. Daily deploys having larger
deltas that often span a weekend. Both builds tend to use a recent
changeset, often master.

With this change the daily deploy will use the staging build's
workspace. There is usually a small delta a daily and a recent staging
build so it should be much faster than building a day's changes.

Test Plan:
I tested this on an actual deploy and found that the non-push parts of
the daily deploy took about as long as they did in the staging build.

Reviewers: csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D4807