Add Github Actions CI for Nix

[srid]

To merge into #14

I'm using my personal cachix (https://srid-nammayatri.cachix.org/) cache for now (pushing manually from laptop), but eventually once @vimalkumar or somebody else creates the official nammayatri cache, we should switch to it (at which point, the CI can push to cachix automatically).

Regardless, we still need to manually push (made convenient via the flake app, run as nix run .#cachix) from macOS machines because Github Actions has no¹ ARM macOS builder: actions/runner-images#2187

Footnotes

1. Instead of waiting for Microsoft to implement this, we could as well switch to either Jenkins or Hercules CI. ↩

[srid]

It takes ~4mins just to download from the cache¹. After that, the build is relatively fast (no need to build dependencies and -- if the PR has not changed the package or the dependencies -- many of the local packages as well).

Footnotes

1. This is something we could entirely avoid using the Hercules CI or Jenkins SSH slave route, due to /nix store being persistent across CI runs. ↩