Skaffold artifact cache should not include image-name in hash calculation

The container-debug-support images are published using two sets of names: a short-form like `go` as well as a longer-form like `skaffold-debug-go`. This is done [with a profile using JSONPatch](https://github.com/GoogleContainerTools/container-debug-support/blob/970e571d526eb20b6a67cc525b349f617c0664d2/go/skaffold.yaml#L98-L110). The idea was that long-form and short-form images can be built and published using:

skaffold build # step 1: build and push with long-form names
skaffold build -p deprecated-names # step 2: build and push with short-form names

In step 2, Skaffold's artifact caching should detect that there are no source changes and simply re-push the image from step 1.

This no longer works, and instead step 2 builds the images. I suspect we must be using the `build.artifacts[].image` as part of the artifact hash calculation.