Add vtexplain and vtbackup to base docker auto-build

[frouioui]

Description

This PR adds the vtexplain and vtbackup images to the list of images we autobuild on push to main and on new tags.

Testing workflow on my own fork: https://github.com/frouioui/vitess/actions/runs/6579148743

• vtbackup on my DockerHub: link
• vtexplain on my DockerHub: link

I opened #14316 earlier to remove vtbackup entirely, but then ended up thinking keeping it and building as part of the already-existing process would be better.

Related Issue(s)

• Part of Move Docker builds to GitHub Actions #14149

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

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