feat: add Nix flake for reproducible builds

[jmikedupont2]

Adds flake.nix + CI.

[neuralconfusion]

Hi @lucidrains and contributors,

I don't have much expertise with Nix or GitHub Actions, but I wanted to raise a quick red flag on this PR just to be safe.

Besides randomly adding a Nix flake to the repo out of nowhere, this PR is also using unofficial forked GitHub Actions. E.g. instead of the standard actions/checkout@v4, it uses meta-introspector/checkout@v4.

Since this is from a first-time contributor, and I know that running modified/unofficial actions in CI is the current "hype" in supply chain attacks to expose repo secrets et cetera, I just wanted to point this out before the workflow gets approved or merged.

Feel free to ignore if I'm misunderstanding how this is supposed to work! I'm just a newbie trying to teach myself Github/DevOps.