script: Don’t store fs attributes in S3 release files

ping @lmars

This avoids headers like this in S3:

```text
x-amz-meta-s3cmd-attrs: uid:1002/gname:release/uname:release/gid:1002/mode:33204/mtime:1415983624/atime:1415983518/md5:1a32a3dd85edd79127dbf4aca862f94c/ctime:1415983624
```