Use nix for CI

[Munksgaard]

I mean... seriously?

[melsman]

How do you suggest nix should solve the issue? Isn't it identical to pinning ubuntu to 20.04, for instance?

[athas]

The difference is that ten years from now you don't want to use Ubuntu 20.04, but Nix will still be around.

But really, Nix is overkill here. Why not just use mlkit for CI? I hear it's a pretty decent SML compiler.

[athas]

Oh look, there's already install-mlkit.