Skaffold ignore registryMirror for ONBUILD

### Expected behavior
Calc hash for ONBUILD multisteps docker image using registry proxy server not

### Actual behavior
Skaffold calc docker image hash for multisteps docker image only from index.docker.com. Skaffold ignore registryMirror setup for kaniko.

I have setup SKAFFOLD_CACHE_ARTIFACTS but this ignore proxy too

### Information

- Skaffold version: 2.6.3
- Operating system: ubuntu 22

Error from log
```
parsing ONBUILD instructions: retrieving image \"alpine/git\": GET https://index.docker.io/v2/alpine/git/manifests/latest: TOOMANYREQUESTS: You have reached your pull rate limit. You may increase the limit by authenticating and upgrading: https://www.docker.com/increase-rate-limit" subtask=docs task=Build
```