image env flag only appends env vars

[tylerphelan]

kp image save ... --env FOO=BAR will only append the FOO environment variable to the list of vars on the image resource. This may not be the behavior we want.

Kpack issue: https://github.com/pivotal/kpack/issues/895

[sambhav]

We do rely on this behavior currently and instead use --delete-env to delete things that are not needed. This might cause breakages for users who rely on this in CI/CD envs. for eg.