Nix CI with caching and checking of CoqEAL

[palmskog]

This is a memento that we would eventually like to have CI for paramcoq using the Nix Toolbox and Nix Action that incorporates the following features:

• caching using the coq-community cachix
• checking of the downstream CoqEAL project for every PR and merge

One inspiration is how the hydra-battles project currently checks the downstream Goedel project.

Unfortunately, the Nix Toolbox not currently easy to use without in-depth Nix knowledge, so we will need help from @Zimmi48 or @CohenCyril.

[Zimmi48]

Sure, I can open a PR to set things up!