Fix implicit GCR auth

[imjasonh]

Description

This partially reverts cf4822c, which intended to support GCR auth in scenarios where the credential helper wasn't available (e.g., executor-slim), to preserve backward compatibility since the credential helper used to be present.

However, that change seems to have broken auth to GCR from GCB, and presumably other scenarios where there's implicit GCP auth from the environment.

This change uses ggcr's google.NewEnvAuthenticator to find implicit GCR auth when none other is available, and when the registry being operated against is GCR or AR.

With this change I'm able to run this successfully, which failed before:

cat > Dockerfile < EOF
FROM ubuntu
EOF
docker build -t gcr.io/imjasonh/kaniko -f deploy/Dockerfile .
docker push gcr.io/imjasonh/kaniko
gcloud config set builds/kaniko_image gcr.io/imjasonh/kaniko
gcloud builds submit . -t gcr.io/imjasonh/kaniko-built

(This also works if you build and run the slim image, which does not contain cred helpers 🎉 )

This produced this kaniko-built image: gcr.io/imjasonh/kaniko-built@sha256:6d8fa0158919992232c6f6eaf882ec88df790709384cc84a1043a087f5e34c8a

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Release Notes

Describe any changes here so maintainer can include it in the release notes, or delete this block.

Fix issue with implicit auth against GCR/AR repositories (broken in v1.7.0)