Support pushing cache even when using --no-push

[leosunmo]

Actual behavior
When using --no-push with --cache=true (and --cache-repo=my-reg) Kaniko will not push any cached layers to the registry.

INFO[0007] Pushing layer my-reg/cache/52445215:35656a0e1... to cache now 
INFO[0007] Skipping push to container registry due to --no-push flag 

This is undocumented and a bit unexpected given the presence of the --no-push-cache flag.

Expected behavior
Be able to not push the final image with --no-push but still benefit from cache being pushed.

Additional Information
Our use-case and workflow is that we do not push images immediately after Kaniko builds them, but instead save them as tars (--tar-path) and then scan them with a container vulnerability scanner in the CI pipeline. Once that's passed the tar image is pushed to the registry.

It seems like this used to work before https://github.com/GoogleContainerTools/kaniko/pull/3132/files was merged. It seems to fix an issue (#2752) that probably should have used the --no-push-cache option instead of the --no-push option.

Ideally the cache should be pushed as long as the --no-push-cache flag is not provided, regardless of the --no-push being present or not.

Triage Notes for the Maintainers

Description	Yes/No
Please check if this a new feature you are proposing
Please check if the build works in docker but not in kaniko	N/A
Please check if this error is seen when you use --cache flag
Please check if your dockerfile is a multistage dockerfile

[jability]

The changes that #2752 brought were completely unexpected. I bet a lot of people were using the pushed layers in their pipelines.
I think this issue is the right way to do it.