builder should autocreate cached-tarballs directory

[gbaz]

it assumes its there and we get /home/builder/builder-dir/build-cache/cached-tarballs: getDirectoryContents:openDirStream: does not exist (No such file or directory)

unless we create it by hand.

[gbaz]

(another unrelated thing i notice, using this issue as a catch-all for new small things in the release -- "whether hackage should run the tests" is not good text for the enablement page)