A flake with `inputs.self.submodules = true` breaks when used as an input from GitHub

[pedorich-n]

Describe the bug

If a flake declared with self.inputs.submodules = true is used as an input in another flake with a github:... URL, then the second flake breaks because submodules are not supported by the github fetcher.

Steps To Reproduce

1. Create a simple flake.nix:

{
  inputs = {
    playit-nixos-module = {
      url = "github:pedorich-n/playit-nixos-module?ref=49676e7462a568827315eeded37fc651dd0b9bf9";
      flake = true;
    };
  };

  outputs = _: { };
}

2. run nix flake lock
3. run nix flake lock again
4. Witness error: input attribute 'submodules' not supported by scheme 'github'

Expected behavior

github fetcher should support submodules

Metadata

• nix (Nix) 2.28.4

Additional context

As a workaround, it's possible to switch to git+https://github.com/... scheme, which supports submodules.

(Probably) Introduced by #12421

Also, the issue was discussed in tidalcycles/Tidal#1137 (comment)

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates

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