flake-info only accepts `nixos-*` Git references

[uncenter]

Despite what the docs say:

nixos-search/flake-info/README.md

Line 87 in 081e9e6

It takes any valid git reference to the upstream [`nixos/nixpkgs`](https://github.com/iixos/nixpkgs/) repo as an argument and produces a complete output.

The command does not accept any Git reference, just ones that are valid when prepended with nixos-. This is reflected/implemented here:

nixos-search/flake-info/src/data/source.rs

Line 123 in 081e9e6

"https://api.github.com/repos/nixos/nixpkgs/branches/nixos-{}",

So something like nixpkgs-unstable would lead to a 404 request panic because nixos-nixpkgs-unstable is not a real reference.

So this example provided does not work:

nixos-search/flake-info/README.md

Line 94 in 081e9e6

$ flake-info nixpkgs nixos-21.05

You would need to do flake-info nixpkgs 21.05 instead.

It's an easy fix to remove the nixos- prefix in the source code line highlighted above, however I am not sure what else would need to be changed to ensure the search instance deploys correctly or wherever else this command is used still functions.