to old version of nix in shell.nix

[crop2000]

on master the version of nix is to old to be used with the code provided in src/default.nix (on master)
it fails because fetchGit does not yet suport submodules.

[crop2000]

reproducer in npins checkout folder:

cargo run -- upgrade
cargo run -- add git https://git.lix.systems/lix-project/lix
nix-instantiate --eval npins -A lix.outPath

error: unsupported argument 'submodules' to 'fetchGit', at /home/olaf/projects/rust/forks/npins_project/npins/default.nix:69:16

[crop2000]

@andir you said here that you don't want to update the minimum required version of nix. But it is already broken with current npins.

[andir]

What would be the minimum required version that supports this? Could we throw a warning within the default.nix whenever the version is too old and someone tries to use submodules?

[crop2000]

currently npins is incompatible with 2.3.18 independent from the question if submodule is used.