* guix/scripts/pack.scm (self-contained-tarball)[build](symlink->directives):
Do not add a 'directory' directive for "/". Previously, as soon as we
were using '-S /bin=bin' or similar, we would add every entry a second
time in the tarball; this would translate as hard links in the tarball,
which tar < 1.30 sometimes fails to extract.
Pass symlinks defined in DIRECTIVES to 'tar'.