Don't require the deprecated nixUnstable package

The CI job to create the weekly software update MR failed with

> error: nixVersions.unstable has been removed. For bleeding edge (Nix master,
> roughly weekly updated) use nixVersions.git, otherwise use
> nixVersions.latest.

We don't require unstable Nix anymore, since the experimental `diff-closures`
command we use now is included in stable nix.

Fixes #185.