I'm writing a flake.nix and I'd like to expose a function which takes one argument and then returns a derivation. What would be the correct way of doing this?
I defined:
```
packages.x86_64-linux.test = { input, }: pkgs.writeText "some-file" ''
   ${input}
'';
```

Which i then try to call via:
`nix build --impure --argstr input foo .#test`

But nix complains:
`flake output attribute 'packages.x86_64-linux.test' is not a derivation`