refactor(flake): use nixpkgs lib directly

flake.nix

@@ -75,8 +75,10 @@
   outputs =
     { self, nixpkgs, ... }@inputs:
     let
-      inherit (nixpkgs) lib;
-      flake = import ./flake { inherit lib inputs self; };
+      flake = import ./flake {
+        lib = nixpkgs.lib;
+        inherit inputs self;
+      };
     in
     {
       # Every host under hosts/ is discovered and built.
@@ -96,12 +98,12 @@
         };
 
       # `nix flake check` builds every discovered host toplevel.
-      checks.x86_64-linux = lib.mapAttrs (
+      checks.x86_64-linux = nixpkgs.lib.mapAttrs (
         name: host:
         if host.config.warnings == [ ] then
           host.config.system.build.toplevel
         else
-          throw "Host ${name} has evaluation warnings:\n${lib.concatStringsSep "\n" host.config.warnings}"
+          throw "Host ${name} has evaluation warnings:\n${nixpkgs.lib.concatStringsSep "\n" host.config.warnings}"
       ) self.nixosConfigurations;
     };
 }