Replace ubi9/ubi-minimal with busybox image

[vdemeester]

Changes

Seems like the ubi9/ubi-minimal image may cause e2e failures (at least from time to time).

ubi-minimal or busybox shouldn't really matter for that regression example test, so this switch to an image we already use (as the "shell" image on the controller).

/kind misc

Submitter Checklist

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Release Notes

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[vdemeester]

See https://prow.tekton.dev/view/gs/tekton-prow/pr-logs/pull/tektoncd_pipeline/7349/pull-tekton-pipeline-integration-tests/1724553270275346432 for an example of a failure.

              waiting:
                message: Back-off pulling image "registry.access.redhat.com/ubi9/ubi-minimal:9.0.0-1580"
                reason: ImagePullBackOff

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[afrittoli]

/approve

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[jerop]

/lgtm