.github/workflows: add the "CXL Test Runner" workflow

Convert the upstream run_qemu workflow into a manual, fork-friendly
CXL test runner that can validate arbitrary kernel and ndctl branches
via GitHub Actions.

The workflow is intended to be used by developers who fork the
cxl-test-runner repository and point it at their own kernel and ndctl
work-in-progress branches. The runner builds the kernel, boots it in
QEMU using run_qemu.sh, and executes the CXL unit tests.

Key changes:
- Switch to workflow_dispatch only and add inputs for kernel/ndctl
  repository + branch, plus a configurable timeout.
- Drop the build matrix and pin execution to ubuntu-24.04 with a fixed
  mkosi image (ubuntu/noble).
- Plumb kernel/ndctl inputs into checkout and runtime environment, and
  print the exact revisions under test.
- Rework run_qemu execution to tee serial console output to
  /tmp/rq_0.log and preserve it across failures; add early QEMU/QMP
  sanity checks.
- Add a per-test + totals summary to both step output and the Actions
  run summary, and upload key logs as artifacts.
- Keep ccache support and update the cache keying to match the new
  runner shape.

See README.md for usage instructions and examples of how to trigger
the workflow from a developer's kernel repository.

Signed-off-by: Alison Schofield <alison.schofield@intel.com>

Author: AlisonSchofield