support/download: fix the bzr helper

bzr uses the name of the extension of the output file to known what
output format to use: tar, tgz, tar.bz2... If no extension is
recognised, bzr will output to a directory.

Since we use 'mktemp .XXXXXX' to generate temporary files, it obviously
never ends with a recognised extension. Thus, bzr expects the output to
be a directory, and fails since it is a file.

Fix that by forcing the output format.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>