It appears snapshot target builds have switched from GPG signatures
(sha256sums.asc) to usign signatures (sha256sums.sig).
This adds support for verifying these usign signatures. (GPG signatures
will also be verified if found.)
This also restores the alphabetical ordering of packages to be installed
by apt-get.
Signed-off-by: Jeffery To <jeffery.to@gmail.com>