Hard coded '/tmp' path prevents restore for large archives

[markpryan]

Looking in archive-tar.sh, the value defined for BITBUCKET_RESTORE_ROOT is hard-coded to use the /tmp directory.

I have the issue that our backup archives are huge and the /tmp directory too small. As a workaround, I just modify to a temp directory with enough free disk, but a better solution would be to use /tmp only as a fall back and use something like "TMPDIR" environment variable to allow an override.