chore: add KOCACHE support to CI workflows

[waveywaves]

Changes

Pre-cache ko base images from .ko.yaml to reduce build times and network usage during CI runs.

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[vdemeester]

We need to do more than this. As is it's almost useless. The idea is to use the cache action (and some hash that identifies the cache from the .ko.yaml file), so that we fetch it once, put it in the cache, and then rely on the cache for all other runs (that share the same .ko.yaml).