chore(skaffold): trim dependencies of buildx script The `docker-client` package includes the docker cli, docker-compose, and some other dependencies. It's safe for us to assume that `skaffold-buildx-build` will only be run in environments where `docker` is already available. The `docker-buildx` package is just the `docker-buildx` plugin, which is not in github actions runners by default, so it's necessary.