Test Lean/Coq output when it's all working

[Timmmm]

When the next Sail compiler is released we should update to it and add the Lean output to CI, and ideally Coq too if it works.

Bonus points if we can actually compile/test the Lean/Coq code somehow.

[jordancarlin]

Related to this, it’s probably worth testing compiling the model against the latest head of the sail compiler. Not sure whether we should do that in this repo or over in the sail repo as both potentially need changes as either evolves.

[Timmmm]

I dunno, that means we'd have to deal with OCaml again... I'd rather just have more regular releases of the Sail compiler.

[jordancarlin]

We might need to deal with opam in CI again anyway if we want to actually compile alternative outputs like Coq.