Problem: In a dev checkout, `_nix_submit_cmd` is nil (only baked in by
the nix derivation). The uv fallback fails with code 127 on NixOS because
`uv` is not a bare system binary — it's only available via the FHS-wrapped
`cp-nvim-submit` script produced by `mkSubmitEnv`.
Solution: Add `discover_nix_submit_cmd` mirroring `discover_nix_python`:
runs `nix build #submitEnv --no-link --print-out-paths`, caches the result
in `stdpath('cache')/cp-nvim/nix-submit`, and sets `_nix_submit_cmd`.
`run_scraper` calls `setup_nix_submit_env()` before spawning submit.
Barrett Ruth
690469bd99
