download_file caches original zips in ~/.gerritcodereview/buck-cache.
If this is a different filesystem than $(pwd)/buck-out a symlink
was used to connect the build file with the original archive. This
confuses Buck when creating the $SRCDIR for a genrule().
If a hardlink does not work, copy the file.
Change-Id: If66c59fb6aecc93b5a3e85cdd76ef880538875ff