Add flake output for nix package with flake-schemas support.

[Lehmanator]

None of:

• DeterminateSystems/flake-schemas
• NixOS/nix
• NixOS/nixpkgs

have a package definition that includes flake-schemas, and building a working version isn't trivial for user flakes.

There hasn't been any activity on the PR in NixOS/nix in a while. Not sure if this repo is the appropriate place for this, but it would be nice to have a working nix binary until upstream merges the PR.

[srid]

@shivaraj-bh has a working fork here if that is of interest to anyone:

https://github.com/juspay/omnix/blob/83c53491cb55fb5c1a21da2224390d68b9906ddb/flake.nix#L24-L27