appveyor: Temporarily move to previous image to unblock CI.

appveyor: Temporarily move to previous image to unblock CI.

The current image has OpenSSL issues along with some other PATH-related
issues that break our builds.

Move to the "Previous" variant for now while the AppVeyor team is
figuring this out.

Link: https://help.appveyor.com/discussions/problems/38517-cannot-find-path-copenssl-win64-because-it-does-not-exist
Acked-by: Alin Serdean <aserdean@ovn.org>
Acked-by: Kevin Traynor <ktraynor@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>