Images should be rebuilt with the release tag

[skitt]

The release process currently retags images:

• for the first GA, the last RC images are retagged (to avoid unintended changes being released)
• for any other release, the last images built on the appropriate release branch are retagged

Now that binaries embed version information, this results in misleading information being shown to users, e.g. using subctl show versions. For example in submariner-io/subctl#971, the RC1 images were tagged using the last built image on the 0.16 branch, embedding the commit information from those images instead of 0.16.0-rc1 (e.g. release-0.16-d1b6c9e194f8 for the gateway and route-agent images), and reproducing that for the GA images instead of 0.16.0. As a result, even though the setup is correctly running 0.16.0 images, that is not apparent.

Limiting the risk of releasing images with unintended changes is still desirable, so fixing this isn’t as simple as building the relevant branch.