NixOS test for `ogmios`

[aciceri]

To be done together #31 and #29 probably.

Details about this will depend on how we end up by implementing #29, or we connect to real nodes on some network or we spin up our own testnet in tests (or even two tests for both approaches).

Notice that the second approach cannot be a flake's check since it needs internet access, it can be only an Hercules CI effect.

[brainrake]

fixed by #45