Switch CI to build and test Carvel imgpkg bundles rather than pushing
snapshot builds to a public registry. This allows anyone to to run the
full CI sequence anonymously, until the final publish step which
requires credentials. It also means that the acceptances tests are
running against a common artifact that is built once.

There are three release artifacts now:
- `service-bindings-{version}.yaml`
  `kubectl apply -f`'able config with publicly hosted image
- `service-bindings-bundle-{version}.tar`
  an airgap-ready imgpkg bundle containing the release config, runtime
  images which can be relocated into a private image registry and
  samples
- `service-bindings-package-{version}.yaml`
  a Carvel Package that can be consumed by kapp-controller. The imgpkg
  bundle references by the package is hosted in a public registry

Signed-off-by: Scott Andrews <andrewssc@vmware.com>

Signed-off-by: Scott Andrews <andrewssc@vmware.com>

Signed-off-by: Scott Andrews <andrewssc@vmware.com>