Nix 2.20 stops working with git lfs

[ktrinh-anduril]

Describe the bug
Since Nix release of 2.20, git lfs has stopped working. At least from other tickets, it looks like more functionalities have been moved to libgit which doesn't support git lfs so that might make sense

Steps To Reproduce

1. Have the lines below in git config along with git lfs installed:

[filter "lfs"]
	clean = git-lfs clean -- %f
	smudge = git-lfs smudge -- %f
	process = git-lfs filter-process
	required = true

2. Run this in nix repl:

builtins.fetchGit {                  
  url = "https://huggingface.co/openlm-research/open_llama_3b";                                                                                                                     
  rev = "141067009124b9c0aea62c76b3eb952174864057";            
};

Expected it to fetch the LFS files like it did in 2.19 but it skipped them

[thufschmitt]

Discussed during the Nix maintainers meeting on 2024-02-26.
We won't implement it ourselves, but this is open to contributions.

• A proper LFS implementation was previously blocked on the libgit2 rewrite
  • libgit2 with smudging support can be used to shell out of git-lfs
  • Maybe someday we make/use a library-level implementation, but let's not start with that unless its surprisingly easy
    • Might look like the submodule support
    • We have some gitattributes infrastructure