sbuild always purge session even if set to never.

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Hi,

sbuild: 0.65.2-1ubuntu2

I'm running a wily instance, I installed and configure sbuild to retrieve wily chroots.

I ran sbuild -Ad and the build was unsuccessfull, but schroot -al didn't list any session, it was removed.

I then checked my .sbuildrc, and it was set to:
$purge_build_directory = 'successful';
$purge_session = 'successful';
$purge_build_deps = 'successful';

I changed it to:
$purge_build_directory = 'never';
$purge_session = 'never';
$purge_build_deps = 'never';

launched a watch ls -l /var/lib/schroot/session

and re-ran sbuild,

I saw session being well created in /var/lib/schroot/session, but at the end of the process it got automatically purged.