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).