use pre-built base images

[pdabelf5]

Proposed changes

Use pre-built base images in CI workflow on non-forked workflows. Rebuild base images if they do not already exist.

Also, avoid rebuilding images for Trivy scan, use the image built in the workflow.

Testing

Tested forked workflow, users without access to secrets still use cached layers for plus image builds
https://github.com/nginxinc/kubernetes-ingress/actions/runs/7884962378

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork