deduplicate flavours.yaml

[MrBatschner]

What would you like to be added:

There is a flavours.yaml in gardenlinux/gardenlinux and another one in gardenlinux/glci. One of it (preferably the one in gardenlinux/gardenlinux) should have authority and the other one should be removed.

Why is this needed:

Potentially unnecessary duplication of code and other information across the Garden Linux source repository and the release pipeline repository make maintenance harder and introduces opportunity for error.

[ccwienk]

done: 165b822

as of this change, flavours.yaml will either be lookup up in sibling directory (gardenlinux), or (as before) configurable via env-var.

I plan to:

• re-implement publishing to work in single process / locally via CLI
• re-implement pipeline in single job; pbly using concourse, so we can drop tekton (which we had to introduce due to technical reasons that no longer prevail)