treewide: use new procd sysupgrade $UPGRADE_BACKUP variable

treewide: use new procd sysupgrade $UPGRADE_BACKUP variable

It's a variable set by procd that should replace hardcoded
/tmp/sysupgrade.tgz.

This change requires the most recent procd with the commit 0f3c136
("sysupgrade: set UPGRADE_BACKUP env variable").

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit 641f6b6c26cb9ab5e1198810015e5f4b2b5b34ad)