Wrap NixOS build checks

NixOS build checks significantly slowed down `nix flake show` as it
prints out the derivation names, which for NixOS derivations requires a
large amount of evaluation. By wrapping the derivations, we now give
them trivial names. The NixOS configurations are still built when
running checks as they are a dependency of the wrappers.

builtinModules/nixosConfigurations.nix

@@ -32,8 +32,9 @@ in
     nixosConfigurations = configs;
     checks = foldl recursiveUpdate { } (mapAttrsToList
       (n: v: {
-        ${v.config.nixpkgs.system}."nixos-${n}" =
-          v.config.system.build.toplevel;
+        ${v.config.nixpkgs.system}."nixos-${n}" = v.pkgs.runCommand
+          "check-nixos-${n}"
+          { } "echo ${v.config.system.build.toplevel} > $out";
       })
       configs);
   };