Don't use imagePullPolicy: Always on prow jobs

[aojea]

          Which was from me then. But I, too, must have copied it from somewhere else.

Probably pull-kubernetes-verify: https://github.com/kubernetes/test-infra/blob/ab1dea0a2d9bb1db98ab80b06fbb7de1fd381585/config/jobs/kubernetes/sig-testing/verify.yaml#L20-L21

There's a ton of other hits when grepping for it: https://github.com/search?q=repo%3Akubernetes%2Ftest-infra%20%22imagePullPolicy%3A%20Always%22&type=code

Originally posted by @pohly in https://github.com/kubernetes/test-infra/pull/33866#discussion_r1861679655

[pohly]

Why we want this: imagePullPolicy: Always causes additional overhead when the image has a fixed version and doesn't get updated in the registry. Use it only if the image is a floating reference.

[BenTheElder]

We intentionally used it on some jobs that trial the absolutely latest image, but most jobs shouldn't be using it.

[BenTheElder]

/sig testing

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/lifecycle stale

[pohly]

/remove lifecycle-stale /lifecycle frozen