Demo test competing for port on hydra CI runners

Running the demo test complains about port already in use on the hydra CI runners.

https://github.com/cardano-scaling/hydra/pull/2392/checks?check_run_id=62528380256

This is enabled by the process-compose test flake-parts module options defined here:

https://github.com/Platonic-Systems/process-compose-flake/blob/99bea96cf269cfd235833ebdf645b567069fd398/nix/process-compose/test.nix#L15