[codex] Fix t4a CubeK xtask test selectors

[shinaoka]

Summary

• Update cargo xtask test --ci custom package selectors from old CubeK package names to the t4a-cubek* publish closure.
• Drop non-published crates from xtask CI tests and use the published WGPU backend feature instead of removed cubecl/cpu.
• Keep --ci accepted for existing workflow compatibility.

Root cause

After #2 narrowed the workspace, the post-merge linux test jobs still failed because xtask test --ci selected old package names such as cubek-matmul and requested cubecl/cpu, which is not part of the minimal t4a-cubecl publish set.

Verification

• cargo fmt --all --check
• cargo xtask check lint
• cargo xtask test --ci