NixOS flake CI: eval every host, not just one

In a multi-host NixOS flake, CI that evaluates only one nixosConfiguration's toplevel.drvPath stays green when a module used only by another host or a homeConfiguration breaks. Evaluate every nixosConfigurations and homeConfigurations entry. App config files linked with mkOutOfStoreSymlink are never read during eval, so they need their own validation step if one is wanted.