Use tempdir_short instead of tempdir.

Use tempdir_short instead of tempdir.

After cd64dc42d1e1b03e57e6ba3d316e4f9dec52a78d, a significant
percentage of the buildfarm got unhappy, because pg_basebackup chokes
if it tries to create a tarfile with symlink more than 99 characters
in length. To try to fix that problem, use tempdir_short instead of
tempdir, as we do in pg_verifybackup's 003_corruption.pl.

There's a more complicated workaround for the same issue in
pg_basebackup's 010_pg_basebackup.pl, but I'm not clear whether
there's any reason to do it that way here. For now, let's try this,
to at least get the buildfarm green again.

A better long-term fix would be to figure out how to generate tar
files containing long symlinks, but that will have to wait for
another time.