fix: pass --insecure-registry to ko for plain-HTTP KinD registry

[khrm]

ko >=v0.19 writes SBOMs to the registry over HTTPS by default, which fails against the plain-HTTP KinD registry (registry.local:5000) with: "http: server gave HTTP response to HTTPS client"

Set KO_FLAGS=--insecure-registry in the e2e workflow and thread it through ko resolve calls in e2e-common.sh.

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[khrm]

/kind misc

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