When nightly cache CI jobs fail to access brew these rpm's are not downloaded and therefore some of the images are not built but the cache is still updated and succeeds. The cache is therefore incomplete.
This means that tests (both periodics and presubmits) may not be complete because of the lack of images.

The "|| true" in https://github.com/openshift/release/blob/master/ci-operator/step-registry/openshift/microshift/infra/iso-build/openshift-microshift-infra-iso-build-commands.sh#L59-L63 allows the job to continue.

4.21

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