update nix and add dev shell (#769)

We update the lock on the nix flake and also add a dev shell. This means
you can do `nix build` to build the project and `nix develop` to drop
into a development environment with cmake and clang.

Signed-off-by: Ali Caglayan <alizter@gmail.com>
Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>