`swh graph download` explodes if /tmp is too small

As per title. (And the situation will occur in most cases for most users, as /tmp is not going to be back enough for the sizes we are talking about.) According to @vlorentz, this is due to zstd decompression, rather than temporary files during download (which I confirm appears to be stored in the requested download directory). This should be fixable by changing that step. In the meantime, the fix is setting an explicit TMPDIR envvar for swh graph download. (I'm testing it right now.)