Tags immutability should be enforced by PR tests

[danielvegamyhre]

I recently submitted a PR updating the image digest for a tag: #5220

However, I noticed the image digest for registry.k8s.io/jobset/jobset:v0.1.1 never updated, and after discussion with @BenTheElder I learned tags are immutable.

I think this should be enforced on PRs via tests, rather than silently failing in the image promotion process, in order to avoid confusion.

/kind bug

[BenTheElder]

cc @kubernetes/release-engineering

/sig release

[BenTheElder]

This also means that we have manifests that don't match reality and will never be fixed (!)

We should probably revert any tag mutations once we prevent introducing new ones.

I think the only robust way to do this is to parse the full config before and after the PR commits and diff the config in memory, git diff can be misleading so just grepping the diff will cause false positives when config is modified in otherwise acceptable ways (e.g. inserting a new image into a file).

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/lifecycle stale

[xmudrii]

/remove-lifecycle stale

[BenTheElder]

/lifecycle frozen