Matrix trimming should account for new tags

When a new tag is added to the manifest, and that is the only change for a given build, matrix trimming will end up trimming out a build for that image, preventing that new tag from being published. The new tag will only be published when a change occurs that would cause the image to be built, such as a Dockerfile change or base image update or disabling caching.

In a scenario like this, the trimming logic should account for newly added tags and cause the build job to run so the tag(s) can be applied.

[Example build](https://dev.azure.com/dnceng/internal/_build/results?buildId=2579581&view=results) where this occurs.