Migrate ci_linux_arm64_clang to new dockerfile.

[ScottTodd]

Progress on #15332. This was the last active use of build_tools/docker/, so we can now delete that directory: #18566.

This uses the same "cpubuilder" dockerfile as the x86_64 builds, which is now built for multiple architectures thanks to iree-org/base-docker-images#11. As before, we install a qemu binary in the dockerfile, this time using the approach in iree-org/base-docker-images#13 instead of a forked dockerfile.

Prior PRs for context:

• Add arm64 server support to CI #14372
• [CI] Run ArmSME tests under emulator as part of build_test_all_arm64 #16331

Build time varies pretty wildly depending on cache hit rate and the phase of the moon:

Scenario	Cache hit rate	Time	Logs
Cold cache	0%	1h45m	Logs
Warm (?) cache	61%	48m	Logs
Warm (hot?) cache	98%	16m	Logs

CI history (https://github.com/iree-org/iree/actions/workflows/ci_linux_arm64_clang.yml?query=branch%3Amain) shows that regular 97% cache hit rates and 17 minute job times are possible. I'm not sure why one test run only got 61% cache hits. This job only runs nightly, so that's not a super high priority to investigate and fix.

If we migrate the arm64 runner off of GCP (#18238) we can further simplify this workflow by dropping its reliance on gcloud auth application-default print-access-token and the docker_run.sh script. Other workflows are now using source setup_sccache.sh and some other code.

[ScottTodd]

Phew, the workflow run on this PR got 98% cache hits. My prior test only got 61%. Updated the table in the PR description.