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 (closed).