[stub] automatically make a pre-release based on main

[enj]

To make it easier to test out the latest bleeding edge code, every merge to main should update a pre-release that could used like so:

kubectl apply -f https://get.pinniped.dev/main/install-pinniped-concierge.yaml

This would of course be an unsupported build.

[anjaltelang]

Reviewed on 12-7-21. Keeping it for now but this is lower in priority. Need to re-assess if we should do this or just document it.