Matches https://github.com/ilyagr/pijul-clone as of now, might become out of sync

flake.nix: Use custom derivation for crate2nix

The default.nix in crate2nix brings in its own version of cargo and LLVM. This change allows us to use just the cargo2nix binary, which reduces the closures-size of the devShell considerably.

Created by  jwaksbaum  on November 17, 2020
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