test(cts): Move some config from actions yaml to xtask

[andyleiserson]

Reduces the number of things that have to be set up manually in order to reproduce CI results locally.

There's still the issue of the default backend on Windows being Vulkan. I considered having cts_runner default to dx12 on Windows, but I haven't done that, not sure whether it makes things more or less confusing.

Testing
Tested locally and will be by CI as well, although it wouldn't hurt to sanity-check the log messages in the CI runs.

Squash or Rebase? Squash

Checklist

• I self-reviewed and fully understand this PR.
• WebGPU implementations built with wgpu may be affected behaviorally.
• Validation and feature gates are in place to confine behavioral changes.
• Tests demonstrate the validation and altered logic works.
• CHANGELOG.md entries for the user-facing effects of this change are present.
• The PR is minimal, and doesn't make sense to land as multiple PRs.
• Commits are logically scoped and individually reviewable.
• The PR description has enough context to understand the motivation and solution implemented.