Warn when nixos stable version is outdated

[imincik]

We can try to raise evaluation warning and set evaluation warnings as errors to make them more visible.

[jfly]

Discussed with @phanirithvij. He found NixOS/nixpkgs#20182, which is quite relevant.

We also learned that nixos includes a SUPPORT_END field in /etc/os-release, which hostnamectl understands:

[root@makemake:~]# hostnamectl
...
    Operating System: NixOS 26.05 (Yarara)
         CPE OS Name: cpe:/o:nixos:nixos:26.05
      OS Support End: Thu 2026-12-31
OS Support Remaining: 5month 3d

Also, the node exporter includes this, you can see a graph of "how long until we're unsupported" here: https://prometheus.nixos.org/query?g0.expr=node_os_support_end_timestamp_seconds+-+time%28%29&g0.show_tree=0&g0.tab=graph&g0.range_input=8w&g0.res_type=auto&g0.res_density=medium&g0.display_mode=lines&g0.show_exemplars=0

[imincik]

Checking support status via prometheus sounds as a right way to go.