Fix helper images being published with the wrong architecture

What does this MR do?

Uses the --platform argument when importing archived tarballs for the various gitlab-runner-helper platforms. If the --platform argument is not passed, the image will properly have the architecture-specific content, but the image metadata will flag it as the default architecture for the runner. Generally linux/amd64.

Why was this MR needed?

With the current system, various container runtimes may refuse to even run the non-amd64 images, because the container metadata advertises them as being for linux/amd64, regardless of their internal content.

What's the best way to test this MR?

I'm not sure if any non-tag builds trigger the ci/release_helper_docker_images script. I tested by simply sourcing ci/docker_commands and verifying import worked as expected with 2 or 3 arguments. But, the best test would be to run the full ci/release_helper_docker_images process and inspect the resulting docker images to ensure their OS, Architecture, and/or Variant match expectations.

What are the relevant issue numbers?

Resolves #30869 (closed).