Use Flake to package Spago for NixPkgs

[peterbecich]

TODO for myself

This new Spago is not yet in NixPkgs.

It should be possible to use the Flake to package Spago for NixPkgs.

The Flake currently doesn't build Spago afaik. This is also needed.

[f-f]

There is already a draft PR that is addressing this: #981