Issue:
chore(ci): Add nix workflow

The builds will run on Juspay managed self-hosted runners (NixOS & nix-darwin machines in office).

## Motivation and Context

To make sure that `flake.nix` remains usable over time and works for users.



Resolution:
<think>
PR Context:
- Title: chore(nix): Unbreak `flake.nix`
- Merged: 2024-09-25T11:16:24Z
- Files changed: 4 files
- Commits: 5 commits

Implementation approach:
Resolves #5876 

This PR makes sure that `flake.nix` builds, and checks it in CI (uses self-hosted runners) going forward. It removes no longer used Nix code (they don't build).

- [x] Add github workflow action to build flake outputs
- [x] Remove packages that no longer build (see #5017)
</think>

To implement this, you'll need to modify:


(No detailed commit information available)