dist/ubuntu: Use tilde for release candidate builds
dist/ubuntu: Use tilde for release candidate builds

The version number ordering rules are different for rpm and deb. Use
tilde ('~') for the latter to ensure a release candidate is ordered
_before_ a final version.

Message-Id: <1b64430fb21e3f31eb0c6e424248a6976dd46b30@scylladb.com>

kjniemi/scylla,scylladb/scylla,duarten/scylla,duarten/scylla,kjniemi/scylla,kjniemi/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,raphaelsc/scylla,scylladb/scylla,duarten/scylla,raphaelsc/scylla,raphaelsc/scylla,scylladb/scylla,avikivity/scylla