Update cachix in CI

[juliankuners]

The release workflows are failing in k, evm-semantics, and possibly in other repositories on MacOS machines when pushing to a nix cache.

The exception means that the peer, cachix, closed the HTTP connection:

(InternalException Network.Socket.recvBuf: resource vanished (Connection reset by peer))

This PR is an attempt to fix this by updating cachix in CI from version 14 (Jan 12, 2024) to version 16 (Mar 10, 2025).

[juliankuners]

The workflows install the newest version anyway by running nix-env --quiet -j8 -iA cachix -f https://cachix.org/api/v1/install internally. On top of that, the MacOS CI machines skip running the cachix workflow. Instead, the MacOS machines use their own locally installed cachix, which on rvwork-m1, e.g., is on version 1.7.9.