Add Nix support and CI

[W95Psp]

This PR:

• add support for building via Nix;
• changes a few things the Makefile to make it a bit more flexible;
• add two github actions (one for PRs and pushes, the other for building/testing Eurydice every day against latest versions of F*/Karamel/Charon).

This PR requires https://github.com/FStarLang/karamel/pull/405. Currently, the karamel branch nix-expose-lib is hardcoded: I will rebase once https://github.com/FStarLang/karamel/pull/405 is merged.

Note: the first build will be slow, and then things will get cached!

[W95Psp]

Nice, will do! :)

Yes that would indeed be great! However, I'm wondering whether this Kyber workflow should live on this repo or on libcrux' repo? Or both maybe. I guess the main point for Kyber is to catch Kyber changes that would make Eurydice produce bad code: for that matter that's better if the CI is yielded at every PR & push on Kyber, right? Another point is to make sure Eurydice is still working while bumping dependencies. So maybe at the end of latest.yaml I can include Kyber extraction and tests, wdyt?

Anyway, indeed this will be for a subsequent PR :)

[msprotz]

Both of your suggestions sound good! I'll happily take the subsequent PRs when you have time. Thanks so much