Nix pre-commit and ci checks

Currently, the nix lock files (for crate2nix and for the packages via niv) are only updated when one of the Nix users runs:

• make regenerate-nix, and
• niv update

This means that dependencies in Cargo.lock and Cargo.nix might be at different versions.

To resolve this, we can:

• Add the update commands to the pre-commit hooks, and/or
• Run the commands in CI and check for changes

Optionally, perhaps make regenerate-nix could also do the niv update so there is one command.