Run on ARM64

Use a GitLab-compatible ARM64 tag for ARM jobs.

arm64-tests:
  stage: test
  tags:
    - saas-linux-medium-arm64
  script:
    - test "$(uname -m)" = "aarch64"
    - ./test-arm64.sh

If you need only architecture matching and not a SaaS shape, use arch:aarch64.