ci: a CmuxNext suite lane never waits for an active cmux-tui artifacts run - #27454

One shadow release compile runs at a time and one waits; another dispatch now would evict the waiting one. The scheduled pick compiles the newest feat-cmux-next head when the slot frees, and a PR's fallback compile has its own group.
Source SHA: e34192c38f1f6012ffd1f8d2706bccf4ff8fceb2