Update docker-credential-gcr to use /v2

[chauncey-garrett]

The docker-credential-gcr go package is outdated and pinned to a version that's now roughly 8 months old (v2.1.8). In this code comment, there's a linked issue regarding the use of /v2 of the go package that's since been resolved.

The docker-credential-gcr repo's docs mention using

go get -u github.com/GoogleCloudPlatform/docker-credential-gcr/v2

It'd seem prudent to update to this for clarity. The latest of docker-credential-gcr is currently v2.1.22.

https://github.com/GoogleCloudPlatform/docker-credential-gcr/releases/tag/v2.1.22

It's also worth mentioning that we're currently unable to auth to GAR using kaniko's installed version of docker-credential-gcr. If we exec into a kaniko container and overwrite the docker-credential-gcr binary with the latest version, our issue goes away.

wget -qO- "https://github.com/GoogleCloudPlatform/docker-credential-gcr/releases/download/v2.1.22/docker-credential-gcr_linux_amd64-2.1.22.tar.gz" | tar xz docker-credential-gcr && \
    chmod +x docker-credential-gcr && \
    mv docker-credential-gcr /kaniko/

[aaron-prindle]

Thanks for flagging this, submitted #3026 to resolve this.

[chauncey-garrett]

Thank you for quickly fixing this!