Support private registries for building the artifacts using buildpacks

### Expected behavior

Building the artifacts should support buildpacks coming from a private registry.

### Actual behavior

It's not possible to define a registry for the buildpacks.

A suggestion for how it could look like:

skaffold.yaml (see line 6)
```yaml
build:
  artifacts:
  - image: imagename
    buildpacks:
      builder: paketobuildpacks/builder:base
      registry: private-registry
```

Currently, you can configure pack for that private registry in the following way:
`pack config registries add private-registry https://...`

### Information

- Skaffold version: 2.3.0
- Operating system: macOS M1
- Installed via: Homebrew