Problem: In a dev checkout on NixOS, `_nix_submit_cmd` is nil but
`_nix_python` is set from the discovery cache. `get_python_submit_cmd`
fell through to `get_python_cmd`, which returned the nix-built Python —
a derivation that deliberately excludes `scrapling`.
Solution: Fall back to `uv run` instead of `get_python_cmd` so submit
always gets a full dependency environment when `_nix_submit_cmd` is nil.
Barrett Ruth
128ff04621
