CI: Initialize preflight.image to match agent.image
CI: Initialize preflight.image to match agent.image

cilium-preflight's primary purpose is to preload the cilium images into
the cluster, so the image used should always match the target agent
image.

The update/upgrade tests attempt to mimic the instructions
we give for upgrades but did not set `preflight.image`. This resulted in
the cilium:latest being used (or other arbitrary combiniations with the
value of `global.tag`) and the preflight load step would fail.

We now initialize, globally, `preflight.image` to match `agent.image`.
This allows helm generation for preflight to select the correct image.

Signed-off-by: Ray Bejjani <7f525154e857f3d57a641a83dddc584f42addc6a@isovalent.com>

tklauser/cilium,tgraf/cilium,michi-covalent/cilium,tklauser/cilium,tgraf/cilium,tgraf/cilium,cilium/cilium,michi-covalent/cilium,cilium/cilium,michi-covalent/cilium,tgraf/cilium,tgraf/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,tklauser/cilium,tklauser/cilium,tgraf/cilium,cilium/cilium,cilium-team/cilium,tklauser/cilium,michi-covalent/cilium,cilium-team/cilium