[DankActions] Integrate with Nix

It would be helpful to have DankActions compatible with Nix to have a declarative way to have scripts running output in the bar. So far I could track the output to a `variant_$id`, but that number appears to be difficult to track to link in a `.nix` derivation.

Start by examining DankActions' variant_$id output tracking and how the current scripts connect their output to the bar. Determine what a Nix-compatible declarative integration would need to expose, then verify that scripts defined through a .nix derivation can run and display their output without manually tracking the variant number.

Written by the indexing model from the issue text.