fixes reference to nixpkgs in pre fetch command

[t-monaghan]

Summary

Adding a flake reference is reporting an error when loading the devbox shell, however everything works as expected.

The offending internal command is the prefetch here: https://github.com/t-monaghan/devbox/blob/eed56cd6e516bb86577c9abcaf421fe88c714296/internal/nix/nixpkgs.go#L40-L52

And the reported error I receive is: error: getting status of '/Users/tom.monaghan/dev/scratch/nixpkgs/d7600c775f877cd87b4f5a831c28aa94137377aa': No such file or directory

This is due to the prefetch missing the github: prepended to the flake reference, and as such nix goes looking for a local copy of nixpkgs.

How was it tested?

With a devbox.json containing the below, I ran the build for this change and noticed no raised error, and the claude-code package I expected was available.

{
  "$schema": "https://raw.githubusercontent.com/jetify-com/devbox/0.16.0/.schema/devbox.schema.json",
  "packages": [
    "github:NixOS/nixpkgs/nixos-unstable#claude-code"
  ],
  "shell": {
    "init_hook": [
      "echo 'Welcome to devbox!' > /dev/null"
    ],
    "scripts": {
      "test": [
        "echo \"Error: no test specified\" && exit 1"
      ]
    }
  }
}

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