fix(release): preserve tag prefixes when retagging promoted images - #3895

…4460)

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## What this PR does

A PR stacked on a feature branch now overlays the image refs of the line
it grew from, so its E2E no longer falls back to the refs committed in
the tree.

The Finalize job pulls `cozystack-packages:<base branch>` and overlays
its image refs onto the packages the PR did not rebuild. For a stacked
PR the base is something like `fix/foo`. The slash is not valid in an
OCI tag, and no workflow publishes an artifact for that branch anyway.
So the pull failed, the step only printed a warning, and every unbuilt
package ran its committed release ref. That is how stacked PRs got the
Grafana plugin 404 described in #4391.

Now `main` and `release-X.Y` bases read their own artifact. Any other
base reads the artifact of whichever of main and the release lines it
has the fewest commits on top of, and a notice says which one. A stack
on a release line keeps that line's images, for example #3895, which
sits on `release-1.5`. On a tie main wins, so a stack on a line cut from
main reads main's artifact until the stack contains a commit of the
line's own. Always taking main there would put main's images under the
line's charts, and that is the install failure from #3437. The new bats
case builds a small git fixture and checks the tag pulled for six bases,
one of them a deleted branch.

The other half of #4391, the stale `grafana-dashboards.tag`, is a
release-managed digest pin and is not touched here. It gets re-stamped
at release prep.

Part of #4391

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- [x] No downstream repository is affected by this change
- [ ] [cozystack/website](https://github.com/cozystack/website) -
follow-up:
- [ ]
[cozystack/terraform-provider-cozystack](https://github.com/cozystack/terraform-provider-cozystack)
- follow-up:
- [ ]
[cozystack/ansible-cozystack](https://github.com/cozystack/ansible-cozystack)
- follow-up:
- [ ] [cozystack/ccp](https://github.com/cozystack/ccp) - follow-up:
- [ ] [cozystack/talm](https://github.com/cozystack/talm) - follow-up:
- [ ] [cozystack/cozyhr](https://github.com/cozystack/cozyhr) -
follow-up:
- [ ] [cozystack/cozy-proxy](https://github.com/cozystack/cozy-proxy) -
follow-up:
- [ ]
[cozystack/cozystack-telemetry-server](https://github.com/cozystack/cozystack-telemetry-server)
- follow-up:
- [ ]
[cozystack/external-apps-example](https://github.com/cozystack/external-apps-example)
- follow-up:
- [ ] [cozystack/examples](https://github.com/cozystack/examples) -
follow-up:
- [ ] [cozystack/community](https://github.com/cozystack/community) -
follow-up:

### Release note

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```release-note
fix(ci): E2E for a PR stacked on a feature branch overlays the image refs of the line the stack grew from, instead of falling back to the committed release refs.
```