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:

test-infra/config/jobs/kubernetes/sig-testing/verify.yaml

Lines 20 to 21 in ab1dea0

	- image: gcr.io/k8s-staging-test-infra/kubekins-e2e:v20241127-f74b74d1f9-master
	imagePullPolicy: Always

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 #33866 (comment)

[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