rustfmt versions differ between nix shell and flake check

[VAWVAW]

On main (after fixing the rust-toolchain.toml hash) running cargo fmt --check inside a nix develop shell prints warnings about rustfmt not being the nightly version but it succeeds.

But running nix flake check prints error about incorrect formatting.

How do we get the same rustfmt version in the flake check and the dev shell?

@benjajaja

[benjajaja]

VAWVAW#1

[VAWVAW]

Fixed in #545