Get Nix Flake to include git submodule

[Adda]

For future reference, one approach since Nix 2.27.0 is to use the inputs.self.submodules flake attribute:

@edolstra

inputs.self.submodules flake attribute #12421

Flakes in Git repositories can now declare that they need Git submodules to be enabled:

{
  inputs.self.submodules = true;
}

Thus, it’s no longer needed for the caller of the flake to pass submodules = true.