Include whether images will be pushed in `build --dry-run` output

In response to #3546, #4039 added a `skaffold build --dry-run` to output the list of fully-qualified tagged image references that would result from a build.

```
$ skaffold build -q --dry-run
{
"builds": [
{
"imageName": "leeroy-web",
"tag": "leeroy-web:v1.32.0-23-gbc3824bc7"
},
{
"imageName": "leeroy-app",
"tag": "leeroy-app:v1.32.0-23-gbc3824bc7"
},
{
"imageName": "base",
"tag": "base:v1.32.0-23-gbc3824bc7"
}
]
}
```

It would be useful if this included whether the images will be pushed, or loaded into a local cluster (avoiding a push).

My suggestion is that we add a new boolean field called `push`.