[release-1.8] 🌱 Use shellcheck binary instead of self-built docker image

[k8s-infra-cherrypick-robot]

This is an automated cherry-pick of #2211

/assign sbueringer

[k8s-ci-robot]

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[sbueringer]

Let's resync the whole thing in one PR per branch

/close

[k8s-ci-robot]

@sbueringer: Closed this PR.

In response to this:

Let's resync the whole thing in one PR

/close

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