Instead of caching downloaded files in `./buck-cache` in the
working folder, cache them in `.gerritcodereview/buck-cache` in
the user's home folder.
This offers two advantages:
- Cached files are not lost if the working folder is removed, or
untracked files are removed with `git clean`.
- The same cached files can be shared between multiple working
folders, i.e. if the developer has separate workspaces to work
on different target branches.
Change-Id: I3e132df85c43431c2454421b1de0d9e901679540
45286f196a