Fix a couple of Nix warnings

See merge request !543

Save some disk space on VMs

Closes #189

See merge request !542

These turned up in https://whetstone.private.storage/privatestorage/PrivateStorageio/-/jobs/24753

bump nixpkgs version

See merge request !541

Monitoring: fix misleading wording in network link dashboard.

See merge request !539

bump nixpkgs version

See merge request !538

bump nixpkgs version

See merge request !535

Don't require the deprecated nixUnstable package

Closes #185

See merge request !533

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.

bump nixpkgs version

See merge request !531

bump nixpkgs version

See merge request !529

Improve services "RED" dashboard

See merge request !527

bump nixpkgs version

See merge request !526

bump nixpkgs version

See merge request !524

Explicitly ask Nix to not install any docs

Closes #180

See merge request !522

bump nixpkgs version

See merge request !521

Upgrade node-exporter-full dashboard to use timeseries panels

See merge request !519

... using curl to get latest version from
https://grafana.com/grafana/dashboards/1860-node-exporter-full/
and adopting it to use our named Prometheus data source

bump nixpkgs version

See merge request !518

Less false positive alerts on monthly backup checks after reboot

See merge request !517