Publish `tkn` image on ghcr (on released and commits)

[vdemeester]

We should maintain an "official" tkn image here, using GitHub packages (aka ghcr.io). The idea would be:

• Any commit in master pushes an image (with a floating tag nightly)
• On release, we push tagged release as well

cc @tektoncd/cli-maintainers

[vdemeester]

This would remove the need to maintain and update a tkn image in tektoncd/plumbing