On many Linux distributions (including openSUSE) the OS env variable is
already used with value "Linux". This results in weird errors like:
==> .1: Box 'generic/Linux' could not be found. Attempting to find and install...
To avoid the conflict, use the more neutral and less likely to be taken
DISTRO variable.
Signed-off-by: Michal Rostecki <mrostecki@opensuse.org>