flake: checks: convert to 'flake-parts'

flake/checks.nix

@@ -1,16 +1,20 @@
-{ self, pre-commit-hooks, ... }:
-system:
+{ self, inputs, ... }:
 {
-  pre-commit = pre-commit-hooks.lib.${system}.run {
-    src = self;
+  perSystem = { system, ... }: {
+    checks = {
+      # NOTE: seems like inputs' does not output the 'lib' attribute
+      pre-commit = inputs.pre-commit-hooks.lib.${system}.run {
+        src = self;
 
-    hooks = {
-      nixpkgs-fmt = {
-        enable = true;
-      };
+        hooks = {
+          nixpkgs-fmt = {
+            enable = true;
+          };
 
-      shellcheck = {
-        enable = true;
+          shellcheck = {
+            enable = true;
+          };
+        };
       };
     };
   };

flake/default.nix

@@ -26,8 +26,6 @@ let
       default = self.apps.${system}.diff-flake;
     };
 
-    checks = import ./checks.nix inputs system;
-
     devShells = import ./dev-shells.nix inputs system;
 
     packages = import ./packages.nix inputs system;
@@ -74,6 +72,7 @@ flake-parts.lib.mkFlake { inherit inputs; } {
   systems = mySystems;
 
   imports = [
+    ./checks.nix
     ./lib.nix
     ./nixos.nix
     ./overlays.nix