Create separate dockerfile-builder-arm64 tag for init_docker_images()

[bensze01]

init_docker_images() uses a separate node label for x86 docker images, which allows us to bypass queue contention with all.sh components when creating / checking the docker images.

When we added the arm64 images they were not used by all.sh components, so we didn't set up a separate label - but now these components are creating a bottleneck.

Create equivalent helper labels for arm64, and also consider movingget_branch_info() to helper nodes.

The labels are defined in the following bundle:
(Internal link): https://gitlab.geo.arm.com/software/eng-infra/oss/tf-openci/cloudbees-bundles/-/blob/main/openci-production-prod/jenkins-clouds.yaml?ref_type=heads